This commit adds a pointer value and two methods (mAttachedToObj,
setAttachedToObj() and getAttachedToObj()) to the SceneObject class.
mAttachedToObj is how attachable classes identify themselves to objects
that are attached. Player class objects need this notification so they
won't warp their position while attached. This value does not need
networked. The attachable class is responsible for setting it on the
server and clients.