Added a Scene2DDebugRenderer

A lot of times I see newbies implementing custom Actors and making mistakes like not setting any size, or setting a position but rendering a (for example) button texture to some different coordinates.

I found myself in need for something like this as well when I was implementing an "overlay input layer" for box2d bodies for example. This "layer" was completely invisible, but it would be nice to be able to easily enable some kind of outlines for the actors to see where the touch areas would be.

Do you think it is useful? Any suggestions for additional features/changes?
I didn't have a look at the refactored code yet, but the custom `Actor` should free them, whenever they want to. I wasn't able to free them automatically (for example after rendering them in `Scene2DDebugRenderer`, because `Table` keeps them for a longer time.