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_ios.go

@@ -138,7 +138,6 @@ func onCreate(view, controller C.CFTypeRef) {
 	w.displayLink = dl
 	C.gio_viewSetHandle(view, C.uintptr_t(cgo.NewHandle(w)))
 	w.Configure(wopts.options)
-	w.ProcessEvent(StageEvent{Stage: StageRunning})
 	w.ProcessEvent(UIKitViewEvent{ViewController: uintptr(controller)})
 }
 
@@ -189,14 +188,12 @@ func (w *window) draw(sync bool) {
 func onStop(h C.uintptr_t) {
 	w := viewFor(h)
 	w.hidden = true
-	w.ProcessEvent(StageEvent{Stage: StagePaused})
 }
 
 //export onStart
 func onStart(h C.uintptr_t) {
 	w := viewFor(h)
 	w.hidden = false
-	w.ProcessEvent(StageEvent{Stage: StageRunning})
 	w.draw(true)
 }