Make X11 dependency optional

Currently Mutter can't be built as a Wayland only compositor. The goal of this issue is to track the various tasks in order to achieve that.

• Make XWayland optional
  PR: !2399 (merged)

• Move X11 windows frames to a separate client
  PR: !2175 (merged)

• Remove GTK usage from libmutter as it brings X11 dependencies indirectly from GDK -> GDK X11
  It would require a small patch to GNOME Shell (the infra from Mutter side is already handled on the PR below) so that the producer/consumer for key events is Mutter and no longer GTK.
  Depends on: !2175 (merged)
  PR: !2407 (merged)
  This PR would also make GDK a X11 specific dependency

• Cursor loading: Currently mutter uses xcursor which handles loading the cursor assets from the user theme. There is no replacement. We could have an in-tree copy of what we need from xcursor. GTK 4 seems to do a similar thing, see https://gitlab.gnome.org/GNOME/gtk/-/blob/main/gdk/wayland/cursor/xcursor.c.
  Using libwayland-cursor is not an option here as it doesn't allow us to get access to the internal Cursor type and rather give us a wl_buffer so we can't share the same code between X11/Wayland backends. At the end of the day, it also has an in-tree copy of xcursor.

• Use GLES instead of GL, it seems GL brings back x11 through GLX. It should be a matter of replacing gl pkg-config with glesv2. It should probably be egl

• Keybindings: Couldn't come up with an idea by looking at the code, maybe we could have a common base class and have different implementation per backend instead of a bunch of if(!is_wayland_compositor()) return all over the place

• jadahl has done some work as part of !2392 (closed) to split the X11/Wayland Barrier implementations.

• Command Line: Error out when Wayland/X11 specific backends options are used and the corresponding backend is disabled at build time

• Untangle _meta_window_shared_new, as it takes x11/wayland specific arguments and there should probably be two variants and a post construction common function

• Add a CI job for the new build configurations:

  • Wayland only
  • X11 only

• Error out if startup-notification is enabled but not x11

The remaining task would be to ensure that we are properly disabling X11/XWayland specific code

I have already managed to do a quick build without X11 stuff

ldd  _build/src/mutter
	linux-vdso.so.1 (0x00007ffe6853d000)
	libmutter-10.so.0 => /var/home/bilelmoussaoui/Projects/mutter/_build/src/libmutter-10.so.0 (0x00007f223c68f000)
	libglib-2.0.so.0 => /lib64/libglib-2.0.so.0 (0x00007f223c548000)
	libgio-2.0.so.0 => /lib64/libgio-2.0.so.0 (0x00007f223c36d000)
	libgobject-2.0.so.0 => /lib64/libgobject-2.0.so.0 (0x00007f223c30e000)
	libwayland-client.so.0 => /lib64/libwayland-client.so.0 (0x00007f223c2fc000)
	libc.so.6 => /lib64/libc.so.6 (0x00007f223c0f9000)
	libmutter-cogl-10.so.0 => /var/home/bilelmoussaoui/Projects/mutter/_build/src/../cogl/cogl/libmutter-cogl-10.so.0 (0x00007f223c055000)
	libmutter-clutter-10.so.0 => /var/home/bilelmoussaoui/Projects/mutter/_build/src/../clutter/clutter/libmutter-clutter-10.so.0 (0x00007f223bf0c000)
	libgraphene-1.0.so.0 => /lib64/libgraphene-1.0.so.0 (0x00007f223beed000)
	libwayland-server.so.0 => /lib64/libwayland-server.so.0 (0x00007f223bed7000)
	libEGL.so.1 => /lib64/libEGL.so.1 (0x00007f223bec4000)
	libGLESv2.so.2 => /lib64/libGLESv2.so.2 (0x00007f223beaf000)
	libcairo.so.2 => /lib64/libcairo.so.2 (0x00007f223bd63000)
	libgmodule-2.0.so.0 => /lib64/libgmodule-2.0.so.0 (0x00007f223bd5c000)
	libgdk_pixbuf-2.0.so.0 => /lib64/libgdk_pixbuf-2.0.so.0 (0x00007f223bd2c000)
	libm.so.6 => /lib64/libm.so.6 (0x00007f223bc4e000)
	libcairo-gobject.so.2 => /lib64/libcairo-gobject.so.2 (0x00007f223bc42000)
	libpango-1.0.so.0 => /lib64/libpango-1.0.so.0 (0x00007f223bbd8000)
	libcanberra.so.0 => /lib64/libcanberra.so.0 (0x00007f223bbc4000)
	libxkbcommon.so.0 => /lib64/libxkbcommon.so.0 (0x00007f223bb7b000)
	libgudev-1.0.so.0 => /lib64/libgudev-1.0.so.0 (0x00007f223bb6d000)
	libudev.so.1 => /lib64/libudev.so.1 (0x00007f223bb40000)
	libpipewire-0.3.so.0 => /lib64/libpipewire-0.3.so.0 (0x00007f223ba62000)
	libsystemd.so.0 => /lib64/libsystemd.so.0 (0x00007f223b983000)
	libdrm.so.2 => /lib64/libdrm.so.2 (0x00007f223b96c000)
	libinput.so.10 => /lib64/libinput.so.10 (0x00007f223b918000)
	libgbm.so.1 => /lib64/libgbm.so.1 (0x00007f223b907000)
	libpcre.so.1 => /lib64/libpcre.so.1 (0x00007f223b88d000)
	libz.so.1 => /lib64/libz.so.1 (0x00007f223b873000)
	libmount.so.1 => /lib64/libmount.so.1 (0x00007f223b82a000)
	libselinux.so.1 => /lib64/libselinux.so.1 (0x00007f223b7fe000)
	libffi.so.8 => /lib64/libffi.so.8 (0x00007f223b7f2000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f223c8cf000)
	libmutter-cogl-pango-10.so.0 => /var/home/bilelmoussaoui/Projects/mutter/_build/src/../clutter/clutter/../../cogl/cogl-pango/libmutter-cogl-pango-10.so.0 (0x00007f223b7e6000)
	libatk-1.0.so.0 => /lib64/libatk-1.0.so.0 (0x00007f223b7bc000)
	libjson-glib-1.0.so.0 => /lib64/libjson-glib-1.0.so.0 (0x00007f223b78e000)
	libfribidi.so.0 => /lib64/libfribidi.so.0 (0x00007f223b76e000)
	libpangocairo-1.0.so.0 => /lib64/libpangocairo-1.0.so.0 (0x00007f223b75c000)
	libpangoft2-1.0.so.0 => /lib64/libpangoft2-1.0.so.0 (0x00007f223b741000)
	libfontconfig.so.1 => /lib64/libfontconfig.so.1 (0x00007f223b6f2000)
	libGLdispatch.so.0 => /lib64/libGLdispatch.so.0 (0x00007f223b639000)
	libpng16.so.16 => /lib64/libpng16.so.16 (0x00007f223b5fd000)
	libfreetype.so.6 => /lib64/libfreetype.so.6 (0x00007f223b530000)
	libpixman-1.so.0 => /lib64/libpixman-1.so.0 (0x00007f223b482000)
	libjpeg.so.62 => /lib64/libjpeg.so.62 (0x00007f223b3fe000)
	libthai.so.0 => /lib64/libthai.so.0 (0x00007f223b3f3000)
	libharfbuzz.so.0 => /lib64/libharfbuzz.so.0 (0x00007f223b308000)
	libvorbisfile.so.3 => /lib64/libvorbisfile.so.3 (0x00007f223b2fd000)
	libtdb.so.1 => /lib64/libtdb.so.1 (0x00007f223b2e4000)
	libltdl.so.7 => /lib64/libltdl.so.7 (0x00007f223b2d8000)
	libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007f223b2b8000)
	liblzma.so.5 => /lib64/liblzma.so.5 (0x00007f223b28b000)
	libzstd.so.1 => /lib64/libzstd.so.1 (0x00007f223b1d8000)
	liblz4.so.1 => /lib64/liblz4.so.1 (0x00007f223b1b4000)
	libcap.so.2 => /lib64/libcap.so.2 (0x00007f223b1aa000)
	libgcrypt.so.20 => /lib64/libgcrypt.so.20 (0x00007f223b06d000)
	libmtdev.so.1 => /lib64/libmtdev.so.1 (0x00007f223b066000)
	libevdev.so.2 => /lib64/libevdev.so.2 (0x00007f223b048000)
	libwacom.so.9 => /lib64/libwacom.so.9 (0x00007f223b037000)
	libexpat.so.1 => /lib64/libexpat.so.1 (0x00007f223b00b000)
	libstdc++.so.6 => /lib64/libstdc++.so.6 (0x00007f223add7000)
	libblkid.so.1 => /lib64/libblkid.so.1 (0x00007f223ad9e000)
	libpcre2-8.so.0 => /lib64/libpcre2-8.so.0 (0x00007f223ad08000)
	libxml2.so.2 => /lib64/libxml2.so.2 (0x00007f223ab7b000)
	libbz2.so.1 => /lib64/libbz2.so.1 (0x00007f223ab68000)
	libbrotlidec.so.1 => /lib64/libbrotlidec.so.1 (0x00007f223ab5b000)
	libdatrie.so.1 => /lib64/libdatrie.so.1 (0x00007f223ab52000)
	libgraphite2.so.3 => /lib64/libgraphite2.so.3 (0x00007f223ab30000)
	libvorbis.so.0 => /lib64/libvorbis.so.0 (0x00007f223aafe000)
	libogg.so.0 => /lib64/libogg.so.0 (0x00007f223aaf3000)
	libgpg-error.so.0 => /lib64/libgpg-error.so.0 (0x00007f223aacd000)
	libbrotlicommon.so.1 => /lib64/libbrotlicommon.so.1 (0x00007f223aaaa000)