events: Move the overlay_key_only_pressed handling to keybindings.c

Now the switch statement is entirely about window operations.

src/core/keybindings.c

@@ -1860,6 +1860,11 @@ meta_keybindings_process_event (MetaDisplay        *display,
 {
   switch (event->type)
     {
+    case CLUTTER_BUTTON_PRESS:
+    case CLUTTER_BUTTON_RELEASE:
+      display->overlay_key_only_pressed = FALSE;
+      return FALSE;
+
     case CLUTTER_KEY_PRESS:
     case CLUTTER_KEY_RELEASE:
       return process_key_event (display, window, (ClutterKeyEvent *) event);