New focus change handling

New focus change handling

Instead of relying on gdk's antiquated crossing events,
create a new GtkCrossingData struct that contains the
actual widgets, and a new event controller vfunc that
expects this struct. This also saves us from making sense
of X's crossing modes and details, and makes for a
generally simpler api.

The ::focus-in and ::focus-out signals of GtkEventControllerKey
have been replaced by a single ::focus-change signal that
takes GtkCrossingData as an argument. All callers have
been updated.