Component updates

Provides an onStart function for Component, which is called once when the owner (entity) is added, or once when is added later to existing Entity. Since this function is called when entity is added, the order of adding components does not matter anymore. 
Also provides onEntityAdded / onEntityRemoved, to make components be able to respond when entity is added/removed/reparented

Should MoviePlayer, MovieSprite, Director use onStart instead of onAdded?