Filter out events handled before the plugin before they get GTK+

Fix a problem where GTK+ was warning
'gdk_window_set_user_time called on non-toplevel' for every button
press and click on the mutter stage by excluding such events from
GTK+ processing.

Add a boolean return value to meta_compositor_process_event that
indicates whether the event has been handled and should be filtered
out of the event stream and for mutter, base that on the return
value of the plugin's xevent_filter vfunc.

src/core/display.c

@@ -2424,9 +2424,10 @@ event_callback (XEvent   *event,
 
   if (display->compositor)
     {
-      meta_compositor_process_event (display->compositor,
-				     event,
-				     window);
+      if (meta_compositor_process_event (display->compositor,
+                                         event,
+                                         window))
+        filter_out_event = TRUE;
     }
   
   display->current_time = CurrentTime;