Entity: Add isEntityModifierRegistered method to IEntity

 * I wanted to check if a IEntityModifier was already registered.
 * Before there was no way to get this information. The result was to
track the register calls or always remove and re-add the
IEntityModifier.
 * One can possibly also check with a IEntityModifierMatcher in the
future.
Sprite: Added ISprite and ITiledSprite interfaces

 * I need to habe a Sprite interface to create own types of Sprite
classes. 
 * Similar cause for ITiledSprite
 * Orgainzed imports on the way
ISprite: Added missing ISprite and ITiledSprite files