gtkwindow: run ::event and specific handlers for EWMH action triggering events

gtkwindow: run ::event and specific handlers for EWMH action triggering events

This gives an opportunity for implementations to handle these events
differently, instead of hardcoding the WM-triggering behavior.

gtk_window_event() only forwards events for WM management if the event
widget is not the window (ie. caught when bubbling), so is safe to be
called here without triggering gtk_window_handle_wm_event() twice.

This commit is an adaption to master of
https://bugzilla.gnome.org/show_bug.cgi?id=736702#c1 by Cosimo Cecchi.