app: [macOS] send ViewEvents when the NSView is attached to a NSWindow

Instead of sending ViewEvents once at construction and once at destruction,
it's better to send them when the underlying NSView changes attachment.

The main advantage is that we're about to move the destruction and
emitting of DestroyEvent to the NSView's dealloc method. However, the
dealloc will not be called if user code has a strong reference to it
through a non-empty ViewEvent. By sending an empty ViewEvent when the
view is detached, well-behaving users will remove the strong reference.

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

app/os_macos.m

@@ -89,6 +89,7 @@ static void handleMouse(GioView *view, NSEvent *event, int typ, CGFloat dx, CGFl
 	return layer;
 }
 - (void)viewDidMoveToWindow {
+	gio_onAttached(self.handle, self.window != nil ? 1 : 0);
 	if (self.window == nil) {
 		gio_onClose(self.handle);
 	}