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.m

@@ -276,3 +276,9 @@ void gio_viewSetHandle(CFTypeRef viewRef, uintptr_t handle) {
 	GioView *v = (__bridge GioView *)viewRef;
 	v.handle = handle;
 }
+
+void gio_wakeupMainThread(void) {
+	dispatch_async(dispatch_get_main_queue(), ^{
+		gio_dispatchMainFuncs();
+	});
+}