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_windows.go

@@ -39,7 +39,6 @@ type window struct {
 	hwnd        syscall.Handle
 	hdc         syscall.Handle
 	w           *callbacks
-	stage       Stage
 	pointerBtns pointer.Buttons
 
 	// cursorIn tracks whether the cursor was inside the window according
@@ -275,14 +274,6 @@ func windowProc(hwnd syscall.Handle, msg uint32, wParam, lParam uintptr) uintptr
 	case windows.WM_KILLFOCUS:
 		w.focused = false
 		w.ProcessEvent(key.FocusEvent{Focus: false})
-	case windows.WM_NCACTIVATE:
-		if w.stage >= StageInactive {
-			if wParam == windows.TRUE {
-				w.setStage(StageRunning)
-			} else {
-				w.setStage(StageInactive)
-			}
-		}
 	case windows.WM_NCHITTEST:
 		if w.config.Decorated {
 			// Let the system handle it.
@@ -348,15 +339,12 @@ func windowProc(hwnd syscall.Handle, msg uint32, wParam, lParam uintptr) uintptr
 		switch wParam {
 		case windows.SIZE_MINIMIZED:
 			w.config.Mode = Minimized
-			w.setStage(StagePaused)
 		case windows.SIZE_MAXIMIZED:
 			w.config.Mode = Maximized
-			w.setStage(StageRunning)
 		case windows.SIZE_RESTORED:
 			if w.config.Mode != Fullscreen {
 				w.config.Mode = Windowed
 			}
-			w.setStage(StageRunning)
 		}
 	case windows.WM_GETMINMAXINFO:
 		mm := (*windows.MinMaxInfo)(unsafe.Pointer(lParam))
@@ -645,13 +633,6 @@ func (w *window) wakeup() {
 	}
 }
 
-func (w *window) setStage(s Stage) {
-	if s != w.stage {
-		w.stage = s
-		w.ProcessEvent(StageEvent{Stage: s})
-	}
-}
-
 func (w *window) draw(sync bool) {
 	if w.config.Size.X == 0 || w.config.Size.Y == 0 {
 		return