app: [API] remove StageEvent and Stage

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>

app/os_js.go

@@ -115,7 +115,6 @@ func newWindow(win *callbacks, options []Option) {
 	w.Configure(options)
 	w.blur()
 	w.processEvent(JSViewEvent{Element: cont})
-	w.processEvent(StageEvent{Stage: StageRunning})
 	w.resize()
 	w.draw(true)
 }
@@ -195,17 +194,6 @@ func (w *window) addEventListeners() {
 		}
 		return w.browserHistory.Call("back")
 	})
-	w.addEventListener(w.document, "visibilitychange", func(this js.Value, args []js.Value) interface{} {
-		ev := StageEvent{}
-		switch w.document.Get("visibilityState").String() {
-		case "hidden", "prerender", "unloaded":
-			ev.Stage = StagePaused
-		default:
-			ev.Stage = StageRunning
-		}
-		w.processEvent(ev)
-		return nil
-	})
 	w.addEventListener(w.cnv, "mousemove", func(this js.Value, args []js.Value) interface{} {
 		w.pointerEvent(pointer.Move, 0, 0, args[0])
 		return nil