events: Only pass key events to Wayland if focus is on the stage

Even without a compositor grab, key events may still be expected to
be processed by the compositor and not applications, for instance
when using ctrl-alt-tab to keynav in the top bar. On X11, focus is
moved to the stage window in that case, so that events are processed
before they are dispatched by the window manager. On wayland, we need
to handle this case ourselves, so make sure to not pass key events to
wayland in that case, and move the key focus back to the stage when
appropriate.

https://bugzilla.gnome.org/show_bug.cgi?id=758167