app: [macOS] implement custom event dispatching

To get rid of app.Main, we need to control the main thread. The macOS
[NSApp run] must be called on the main goroutine and never yields control.

Implement the escape hatch which is calling [NSApp stop] to force
[NSApp run] to return and allow us to fetch and dispatch events one at a
time.

This change is separate from the larger change removing app.Main to ease
bisecting.

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

app/os_ios.go

@@ -396,5 +396,12 @@ func gio_runMain() {
 	runMain()
 }
 
+func (w *window) wakeup() {
+	runOnMain(func() {
+		w.loop.Wakeup()
+		w.loop.FlushEvents()
+	})
+}
+
 func (UIKitViewEvent) implementsViewEvent() {}
 func (UIKitViewEvent) ImplementsEvent()     {}