merged ^/branches/z
reintegrated branch z:
  - new event propagation scheme using registered event handlers
  - started some work for running time and pausing the game
removed MoveWilly Event -- target object is always known so just call a method instead
fix segfault on dead willy (don't ever delete him)
bugfix: don't unfreeze willy too early when rocks roll off other rocks
checks for moving entities unnecessary because they change board position as soon as the move finished
make time display immediately when game is started
make code for digit image loading clearer (avoid magic number)