Some fixes in scene edition, along with clipping in the engine player

Just a small comment.

If you take a look at what those clip methods do and how we clip in [MockupSceneEditor](https://github.com/e-ucm/ead/blob/master/editor/core/src/main/java/es/eucm/ead/editor/view/widgets/mockup/scenes/MockupSceneEditor.java), I think we will agree that the last implementation is considerably faster. Why not using it?