Adjust ShapeRenderer to avoid vectorial renderings be painted always on ...

...top.

Idea: 
if the ShapeDrawables will only apear in the EditorGameView, we could move all the implementation there, extending the drawChildren() method. This would require the SpriteBatch to be closed before and opened after but would allow us to decide what is drawn first with the SpriteBatch and what is drawn first with the ShapeRenderer, changing the order of the end result not being always on top.

Note: I've made ShapeRenderer static because in the Mockup project we actually create two instances of EngineView (one for ElementEdition and one for SceneEdition), but it wouldn't make any sense to use two ShapeRenderers, which would only use more memory.
Imagine if we had one SpriteBatch per each actor from the Stage...

Fixes #323