ui/app: introduce DestroyEvent for ending the event loop

Replace the StageDead stage with DestroyEvent dedicated to ending
the event loop and, for premature window closes, the error.

Drop the error return from NewWindow; any errors in window creation
will appear as an immediate DestroyEvent with its Err field set.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/app/os_windows.go

@@ -61,6 +61,7 @@ type window struct {
 	width  int
 	height int
 	stage  Stage
+	dead   bool
 
 	mu        sync.Mutex
 	animating bool
@@ -319,7 +320,8 @@ func windowProc(hwnd syscall.Handle, msg uint32, wParam, lParam uintptr) uintptr
 		w.scrollEvent(wParam, lParam)
 	case _WM_DESTROY:
 		delete(winMap, hwnd)
-		w.setStage(StageDead)
+		w.dead = true
+		w.w.event(DestroyEvent{})
 	case _WM_REDRAW:
 		w.mu.Lock()
 		anim := w.animating
@@ -368,7 +370,7 @@ func (w *window) scrollEvent(wParam, lParam uintptr) {
 // Adapted from https://blogs.msdn.microsoft.com/oldnewthing/20060126-00/?p=32513/
 func (w *window) loop() error {
 loop:
-	for w.stage > StageDead {
+	for !w.dead {
 		var msg msg
 		// Since posted messages are always returned before system messages,
 		// but we want our WM_REDRAW to always come last, just like WM_PAINT.