gdk: Add _gdk_x11_event_translator_get_window()

This method can be implemented by event translators so they
return the right window from XGenericEventCookie events, as
ev->xany.window isn't meaningful for these.

GdkEventSource now also uses this to find out the right window
filters to apply.

gdk/x11/gdkeventtranslator.h

@@ -49,6 +49,8 @@ struct _GdkEventTranslatorIface
   void         (* select_window_events) (GdkEventTranslator *translator,
                                          Window              window,
                                          GdkEventMask        event_mask);
+  GdkWindow *  (* get_window)           (GdkEventTranslator *translator,
+                                         XEvent             *xevent);
 };
 
 GType      _gdk_x11_event_translator_get_type (void) G_GNUC_CONST;
@@ -60,6 +62,9 @@ GdkEventMask _gdk_x11_event_translator_get_handled_events   (GdkEventTranslator
 void         _gdk_x11_event_translator_select_window_events (GdkEventTranslator *translator,
                                                              Window              window,
                                                              GdkEventMask        event_mask);
+GdkWindow *  _gdk_x11_event_translator_get_window           (GdkEventTranslator *translator,
+                                                             GdkDisplay         *display,
+                                                             XEvent             *xevent);
 
 G_END_DECLS