Allow adding custom Triggers without having to write a customMapObjectLoader

[nightm4re94]

The CreatureMapObjectLoader provides a convenient, reflection-based way to register custom Creature types, see this example:

    CreatureMapObjectLoader.registerCustomCreatureType(Zombie.class);

This way, the developer doesn't need to write a custom MapObjectLoader when extending the base Creature type (in this case, with a Zombie).
The same behaviour would be desirable for Triggers and, in fact, any other kind of MapObject that is extended frequently.

[nightm4re94]

Maybe we should even consider making MapObjectLoaders generic in general.