0.8.4a1 - Collision Placement

Enables collision placement - collision always has a single collision
tile selected which cannot be deleted. Collision can either be placed or
erased, and integrates with the undo/redo framework of Areas.

The new class, CollisionItem, inherits from EditorItem and represents a
collision rectangle on the map. PlaceCollisionCommand is the command for
placing collision. Custom logic is implemented for changing the render
mode of collision items in MapView (this cannot yet be generalized
because DestView must remain document-agnostic). This functionality
might be implemented into DestView later on with new functions (e.g.,
getCollisionAt()).