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/GioView.java

@@ -65,8 +65,8 @@ public final class GioView extends SurfaceView implements Choreographer.FrameCal
 	private final InputMethodManager imm;
 	private final float scrollXScale;
 	private final float scrollYScale;
+	private final AccessibilityManager accessManager;
 	private int keyboardHint;
-	private AccessibilityManager accessManager;
 
 	private long nhandle;