Bullets behave correctly again.

- All TmxObjects now allow startRect to be set to null to indicate that they do not have a respawn location.
- TmxObject.updateBoundary() will now use the map's current boundary as the object's boundary whenever startRect is null.
Beginning implementation of callback arrays for certain triggers.

- TmxMap:
-- onBoundaryTransitionBegin, ...Update, ...End
-- onCreate, onRestart

- TmxObject:
-- startRect is no longer initialized by default. It is only set by the map loader.

- Boundary:
-- Despawning behavior for boundary transition now handled using new function despawnAll()

- Util:
-- add() and remove() mixin methods for arrays that work similarly to FlxTypedGroup.add() and remove().