gdk/wayland: Add GdkDisplay call to query available globals

gdk/wayland: Add GdkDisplay call to query available globals

The internal known_globals hashtable is used to carry accounting for
interfaces that depend on others (as ordering is not guaranteed), extend
its usage so it also keeps track of unimplemented interfaces (here at
least).

The API call will then use this to allow querying the globals offered by
the compositor, it will be useful to determine whether we can use
text-input protocols or should fallback to other IMs.