We now rely on toplevels receiving and forwarding all the events
the windowing should be able to handle. Event masks are no longer a
way to determine whether an event is deliverable ot a widget.
Events will always be delivered in the three captured/target/bubbled
phases, widgets can now just attach GtkEventControllers and let those
handle the events.
Quite a few applications use GTK_WINDOW_POPUP to create various
temporary windows and place then on screen. That works fine on X11 but
on Wayland there is no global coordinate system for regular surfaces.
If the application is using a gdk temp window and set a parent with
gtk_window_transient_for(), the gdk wayland backend has all it needs to
create a subsurface that can be placed at will by the application.
Bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=759738
