Re-enable X11 support

Adrian Vovkrequested to merge
avovk/launch-x11-desktops into main

For GNOME 49, we wanted to do the following:

  • Disable X11 support by default
  • Keep X11 code so that distros can re-enable it as needed
  • Keep support for launching X11 desktop environments

Unfortunately this is quite difficult. The x11-support switch, when turned off, disables GDM's entire X11 support, including the ability to launch X11 desktop environments. GDM no longer even looks into /usr/share/xsessions to advertise those sessions as available.

I've attempted to surgically remove the parts necessary for launching modern X11 sessions from the overall x11-support switch, while keeping the more horrible parts of X11 support (XDMCP, x11-legacy displays, non-user-session displays, etc) under its control. This is hard, requires some pretty arbitrary X11 code to exist outside of the switch's control, and frankly is just incorrect.

So, for now, let's turn x11-support back on for GNOME 49. For GNOME 50, we'll be able to outright delete the majority of X11 support, leaving only the ability to launch X11 sessions. Then the x11-support switch can remain as-is, to be used on distros that don't ship any X11 desktop environments or don't care to preserve GDM's ability to launch X11 desktops.

This reverts commits ef8cdaa7 and ab085f7d

Part of #989

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