Refactored runtime's Mundus

[Dgzt]

A bit refactor.

I think with this changes who uses this runtime will have less chance of making mistakes.

Currently:

batch = new ModelBatch();

mundus = new Mundus(Gdx.files.internal("mundus"));
mundis.init();
scene = mundus.loadScene("Main Scene.mundus");
scene.batch = batch;

The developer can leave init method, or forget to set batch and these will generate exception.

With changes this is enough:

batch = new ModelBatch();

mundus = new Mundus(Gdx.files.internal("mundus"));
scene = mundus.loadScene("Main Scene.mundus", batch);