app: prepare Window for removal of Main and asynchronous FrameEvents

This is mostly a refactor, but there are two user-visible effects:
- Window.NextEvent may be called even after DestroyEvent is returned.
- Window.Invalidate always wakes up a blocking NextEvent, even when a
FrameEvent cannot be generated.

As a nice side-effect, X11, Wayland and Wasm no longer require separate
goroutines for their window loops.

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

app/os_ios.m

@@ -56,7 +56,7 @@ CGFloat _keyboardHeight;
 - (void)applicationWillEnterForeground:(UIApplication *)application {
 	UIView *drawView = self.view.subviews[0];
 	if (drawView != nil) {
-		gio_onDraw((__bridge CFTypeRef)drawView);
+		onStart((__bridge CFTypeRef)drawView);
 	}
 }