[cpp] Root API

[Aidan63]

Expose the existing hxcpp rooting api in a haxe friendly way, eliminating the need to use untyped or cpp glue code to root and pass haxe objects into void pointers.

Using this the SDL coro example doesn't need the untyped, e.g. SdlDispatcher.dispatch

public function dispatch(obj:IScheduleObject) {
	final event = new Event();
	event.type = id;
	event.user.data1 = RootHandle.create(obj).toVoidPointer();

	SDL.pushEvent(event);
}

Event dispatching in the loop.

final root = RootHandle.fromVoidPointer(event.user.data1);

final obj : IScheduleObject = root.getObject();

root.close();

obj.onSchedule();

HaxeFoundation/hxcpp#1292

[Simn]

Nice, that example looks really good now!