Yet another A* implementation

I know that there are already two PRs for this topic.

- https://github.com/libgdx/libgdx/pull/1972
- https://github.com/libgdx/libgdx/pull/1977

However they both seem to over-complicate it a bit. The idea was based on @badlogic 's comment. Each node can return its neighbours, which makes the implementation a lot more simple.

What do you guys think?