This makes Wayland and X11 no longer call into XKB and libX11 for these
functions but use GDK's own copy of these functions, just like the
win32, quartz and broadway backends.
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
