wayland: emit GDK_SELECTION_CLEAR on owner changes

wayland: emit GDK_SELECTION_CLEAR on owner changes

The wayland backend currently never emits GDK_SELECTION_CLEAR events.
GtkClipboard uses this signal in order to clear the clipboard owner when
the selection is set to something outside the application.
This commit ensures the wayland backend emits GDK_SELECTION_CLEAR before
setting the clipboard owner to NULL, as this means we lost the
selection.

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
https://bugzilla.gnome.org/show_bug.cgi?id=790031