Fixed size related issues. Selection Manager no longer depends on the fr...

...ameoverlay which is now only manipulated by the tools

Fix for issue #99 . 

Simplified a bit the init of pskl.app . The order of instanciation doesn't matter anymore.

No one keeps a cached version of the overlay anymore, a reference to it is passed by the DrawingController to the tools.

The selection tools are now responsible for clearing and filling the overlay. The selection manager only takes care of keeping the current selection in memory.

Waiting for review.