StageEvent served only redundant purposes:
- To detect whether the window has focus. That is covered by
key.FocusEvent.
- To detect whether the window is currently visible. That is covered by
the absence or presence of FrameEvents.
- To detect when the window native handle is valid. That is
covered by ViewEvent.
Signed-off-by: Elias Naur <mail@eliasnaur.com>