core: Ensure ::x11-display-opened is emitted at a time there are handlers

During display initialization, the MetaX11Display would be opened at a time
when we didn't initialize yet the compositor/plugin. The plugin is certainly
interested in ::x11-display-opened happening, so make sure the signal
happens late enough that the upper layers are able to connect to the signal
in advance.

Fixes xsettings/ibus-x11 initialization on the still default "initialize
Xwayland on startup" wayland compositor behavior.

Closes: #771
5 jobs for !792 with wip/carlosg/x11-services-initialization-fix in 6 minutes and 36 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #428574
check-commit-log

00:00:24

 
  Build
passed #428575
build-mutter

00:02:14

passed #428576
build-without-native-backend

00:02:54

 
  Test
passed #428578
can-build-gnome-shell

00:02:42

passed #428577
test-mutter

00:03:16