panel: Listen differently for button-press events

In the panel we listen for button presses on the panel in order to start a drag op of the adjacent maximized window. With the recent changes removing the source detail of events from Clutter, we now can no longer access buttonEvent.source, but instead need to use ClutterStage.get_event_actor(). Since that function expects us to pass a ClutterEvent and not a ClutterButtonEvent, let's listen for the "event" signal instead of the "button-press" signal so that we have a ClutterEvent we can use.