closes#81, closes#82
- Issue #81: Standardized property name to sprite_index across UISprite and UIEntity
- Added sprite_index as the primary property name
- Kept sprite_number as a deprecated alias for backward compatibility
- Updated repr() methods to use sprite_index
- Updated animation system to recognize both names
- Issue #82: Added scale_x and scale_y properties to UISprite
- Enables non-uniform scaling of sprites
- scale property still works for uniform scaling
- Both properties work with the animation system
All existing code using sprite_number continues to work due to backward compatibility.
