Toward a better editor for Ella

If you're using Ella, you have noticed that, sometimes, the editor's look and feel isn't perfect, mainly when you want to resize or rotate an item. So, since last days, I've worked on those problems :
- less bugs (I hope : none !) when we resize an item. Now the lower corner is "sensitive", and I will probably add a function to change the mouse pointer look when it's hover the lower corner.

- less bugs (but with some remaining) in rotation box : there is now 4 sensitive areas and the selection box try to surround correctly the object during rotation :