This is another step towards making GdkDisplayManager backend-agnostic.
Most of the backends profit from this as their atom implementations
where generic anyway - x11 needed that to allow multiple X displays and
broadway, quartz and wayland don't have the concept of displays.
The X11 backend still did things, so I only #if 0'd some code but did
not actually update anything.
For introspection scanning which ends up calling class_init() which in
turn calls into the keybindings code, we can just use the fallback
keyval conversion code.
https://bugzilla.gnome.org/show_bug.cgi?id=696457
