Implemented focus & better UIManager

- A widget can now be focused.
- The ingame menu now properly locks movement.
- UIManager is now fully polymorphic (no more coercions yay !).
All menus are now implemented
No more ugly option types in Game.ml

-Removed all option types in Game.ml
-Game is now a function (not a class)
Merging master
Merge pull request #86 from GL-MPRI-2014/gamemenus

Gamemenus