WIP: x11: Ignore WM_DELETE_WINDOW if not nested

Something(*) might be trying to send a WM_DELETE_WINDOW event to clutter stage window. If we would be an application window, we should treat this as closing the window, but being a window manager, we shouldn't just listen to random clients telling us to shut down.

So to avoid a misfired WM_DELETE_WINDOW sent by some client, simply ignore those requests if we're not running nested.

• It seems that when running GNOME Shell using the X11 backend, opening a couple of terminal windows, and touching (using a touch screen, or emulated touch screen) frantically switching between the windows, showing the alt-tab preview and overview, there sometimes seems to come WM_DELETE_WINDOW events from somewhere.

  I haven't been able to pinpoint where the events come from, but it doesn't seem to be any gtk3 client, or mutter itself. Running the whole session via xscope makes it impossible (or at least very very much harder) to reproduce, probably due to changed timing.

This fixes crashes such as:

 #0  meta_window_get_buffer_rect (window=0x0, rect=rect@entry=0x7ffd7fc62e40) at core/window.c:4396
 #1  0x00007f1e2634837f in get_top_visible_window_actor (compositor=0x297d700, compositor=0x297d700) at compositor/compositor.c:1059
 #2  meta_compositor_sync_stack (compositor=0x297d700, stack=<optimized out>, stack@entry=0x26e3140) at compositor/compositor.c:1176
 #3  0x00007f1e263757ac in meta_stack_tracker_sync_stack (tracker=0x297dbc0) at core/stack-tracker.c:871
 #4  0x00007f1e26375899 in stack_tracker_sync_stack_later (data=<optimized out>) at core/stack-tracker.c:881
 #5  0x00007f1e26376914 in run_repaint_laters (laters_list=0x7f1e2663b7d8 <laters+24>) at core/util.c:809
 #6  run_all_repaint_laters (data=<optimized out>) at core/util.c:826
 #7  0x00007f1e26b18325 in _clutter_run_repaint_functions (flags=flags@entry=CLUTTER_REPAINT_FLAGS_PRE_PAINT) at clutter-main.c:3448
 #8  0x00007f1e26b18fc5 in master_clock_update_stages (master_clock=0x32d6a80, stages=0x4e5a740) at clutter-master-clock-default.c:437
 #9  clutter_clock_dispatch (source=<optimized out>, callback=<optimized out>, user_data=<optimized out>) at clutter-master-clock-default.c:567
 #10 0x00007f1e27e48049 in g_main_dispatch (context=0x225b8d0) at gmain.c:3175
 #11 g_main_context_dispatch (context=context@entry=0x225b8d0) at gmain.c:3828
 #12 0x00007f1e27e483a8 in g_main_context_iterate (context=0x225b8d0, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at gmain.c:3901
 #13 0x00007f1e27e4867a in g_main_loop_run (loop=0x24e29f0) at gmain.c:4097
 #14 0x00007f1e2636a3dc in meta_run () at core/main.c:666
 #15 0x000000000040219c in main (argc=1, argv=0x7ffd7fc63238) at ../src/main.c:534

and

 #0  0x00007f93943c1f25 in raise () at /usr/lib/libc.so.6
 #1  0x00007f93943ab897 in abort () at /usr/lib/libc.so.6
 #2  0x00007f9393e1e062 in g_assertion_message (domain=<optimized out>, file=<optimized out>, line=<optimized out>, func=0x7f93933e6860 <__func__.116322> "meta_x11_get_stage_window",
 #3  0x00007f9393e4ab1d in g_assertion_message_expr ()
 #4  0x00007f939338ecd7 in meta_x11_get_stage_window (stage=<optimized out>) at ../mutter/src/backends/x11/meta-stage-x11.c:923
 #5  0x00007f939339e599 in meta_backend_x11_cm_translate_device_event (x11=<optimized out>, device_event=0x55bc8bcfd6b0) at ../mutter/src/backends/x11/cm/meta-backend-x11-cm.c:381
 #6  0x00007f939339f2e2 in meta_backend_x11_translate_device_event (device_event=0x55bc8bcfd6b0, x11=0x55bc89dd5220) at ../mutter/src/backends/x11/meta-backend-x11.c:179
 #7  0x00007f939339f2e2 in translate_device_event (device_event=0x55bc8bcfd6b0, x11=0x55bc89dd5220) at ../mutter/src/backends/x11/meta-backend-x11.c:208
 #8  0x00007f939339f2e2 in maybe_spoof_event_as_stage_event (input_event=0x55bc8bcfd6b0, x11=0x55bc89dd5220) at ../mutter/src/backends/x11/meta-backend-x11.c:284
 #9  0x00007f939339f2e2 in handle_input_event (event=0x7fff62d60490, x11=0x55bc89dd5220) at ../mutter/src/backends/x11/meta-backend-x11.c:309
 #10 0x00007f939339f2e2 in handle_host_xevent (event=0x7fff62d60490, backend=0x55bc89dd5220) at ../mutter/src/backends/x11/meta-backend-x11.c:413
 #11 0x00007f939339f2e2 in x_event_source_dispatch (source=<optimized out>, callback=<optimized out>, user_data=<optimized out>) at ../mutter/src/backends/x11/meta-backend-x11.c:467
 #12 0x00007f9393e6c39e in g_main_dispatch (context=0x55bc89dd03e0) at ../glib/glib/gmain.c:3179
 #13 0x00007f9393e6c39e in g_main_context_dispatch (context=context@entry=0x55bc89dd03e0) at ../glib/glib/gmain.c:3844
 #14 0x00007f9393e6e1b1 in g_main_context_iterate (context=0x55bc89dd03e0, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../glib/glib/gmain.c:3917
 #15 0x00007f9393e6f0c3 in g_main_loop_run (loop=0x55bc8a042640) at ../glib/glib/gmain.c:4111
 #16 0x00007f9393369a0c in meta_run () at ../mutter/src/core/main.c:676
 #17 0x000055bc880f2426 in main (argc=<optimized out>, argv=<optimized out>) at ../gnome-shell/src/main.c:552

Related: #338 (closed) Closes: #951 (closed)