Add opacity property to all UIDrawable objects
Add visible property to all UIDrawable objects
Add Vector arithmetic helper methods
Create Python base class for UIDrawables, and derive Python UI types from it
Remove or verify the necessity/safety of default constructors
Grid constructor should accept grid_size tuple
Sprite get/set Texture (swap texture)
UIGrid: don't attempt to render every Entity every frame
Add move() and resize() methods to UIDrawable
Add get_bounds() method to all UIDrawable objects
UIGrid & UIEntity Integration; reliable add and remove