app: [macOS/iOS] simplify running functions in the main thread

Using cgo.Handle allows us to pass a reference to a Go function through
the GCD API for running main thread code, saving a goroutine and a channel.

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

app/os_darwin.m

@@ -4,8 +4,8 @@
 
 #include "_cgo_export.h"
 
-void gio_wakeupMainThread(void) {
+void gio_runOnMain(uintptr_t h) {
 	dispatch_async(dispatch_get_main_queue(), ^{
-		gio_dispatchMainFuncs();
+		gio_runFunc(h);
 	});
 }