gesturestylus: Use GtkEventControllerEvent events to track changes

gesturestylus: Use GtkEventControllerEvent events to track changes

We use gtk_gesture_get_last_event() underneath at places that need to
work during ::proximity emission. Since GtkGesture only tracks events
while there are button/touch presses involved, this is not going to
bring the right result there.

Use gtk_event_controller_get_current_event() consistently inside,
which always pokes at the event being handled (which is the correct
intent here).