New3dapi, various changes

Some minor changes and doc updates. The biggest chage is the signature change of ModelBatch#render to allow multiple RenderableProviders to be specified at once.

All changes:
- Add Model#getManagedDisposables() (used by ModelBuilder)
- Check if a disposble is already in the array before adding it
- Set shader in ModelBatch#render instead of after the array is sorted
- Change render signature to allow multiple RenderableProviders at once
- Allow adding with only shader and no light, allow adding arrays.
- Update gdxinvaders and the tests with the above changes
- Remove ModelInstance#copyMaterials and instead copy only the materials actually used.
- Add ModelInstance instance to specify the IDs of the nodes to copy.
- Add BaseShaderProvider (which reuses and disposes shaders)
- Add ModelBuilder.rebuildReferences which recreates the arrays (materials, meshes and meshparts) within a Model based on the nodes.
- Allow to add a Model as a Node with ModelBuilder (should probably clear the disposables in the source)
- Some other helper methods to ModelBuilder.