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_x11.go

@@ -95,7 +95,6 @@ type x11Window struct {
 		// _NET_WM_STATE_MAXIMIZED_VERT
 		wmStateMaximizedVert C.Atom
 	}
-	stage  Stage
 	metric unit.Metric
 	notify struct {
 		read, write int
@@ -437,14 +436,6 @@ func (w *x11Window) window() (C.Window, int, int) {
 	return w.xw, w.config.Size.X, w.config.Size.Y
 }
 
-func (w *x11Window) setStage(s Stage) {
-	if s == w.stage {
-		return
-	}
-	w.stage = s
-	w.ProcessEvent(StageEvent{Stage: s})
-}
-
 func (w *x11Window) dispatch() {
 	if w.x == nil {
 		// Only Invalidate can wake us up.
@@ -879,7 +870,6 @@ func newX11Window(gioWin *callbacks, options []Option) error {
 	C.XMapWindow(dpy, win)
 	w.Configure(options)
 	w.ProcessEvent(X11ViewEvent{Display: unsafe.Pointer(dpy), Window: uintptr(win)})
-	w.setStage(StageRunning)
 	return nil
 }