Stage remove initialize

Note that I'm not very experienced with scene 2d, so I might be missing something. But I noticed that:
- Stage#batch is private final and has a getter, so it might as well be public final.
- Stage#ownsBatch is private final, it doesn't hurt to make it public final.
- Some private arrays in Stage are set on construction and never set again, so they might as well be final.
- ownsBatch depends on whether a SpriteBatch is given, so adding a batch==null check allows for one general constructor.
- Stage#initialize is private and only called on construction, so it might as well be in the constructor.
- Stage#root is private, only set on construction and has a getter, so it might as well be public final.