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 "button-press-event" signal
instead of connecting to vfunc_button_press_event(). This will give us a
ClutterEvent we can work with.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2225>
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