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