flea circus

Copperbox revision 2992.

I've added a subclass for the flea stack - the first level of the game (there might only be one) has the player jumping on to a stack of fleas to make a tower (c.f circus acrobats).

The flea stack is a specialized Group - unfortunately it seems I have to make a subclass of Group (and inherit a big API) rather than wrap a Group as an instance variable of a new class. If I wrap Group, the flea stack appears to miss some draw event fired in the framework which leaves it invisible.