Add exception-raising copy dunders via reusable decorator

[pushfoo]

TL;DR: Use decorator to stub __copy__ and __deepcopy__ per #2074 + add a bunch of tests

Changes

1. Add decorator which adds stubs with informative messages

2. Apply it to the following + add tests:

   • SpriteList
   • BasicSprite + tests for subclasses:
     • Sprite
     • SpriteSolidColor
     • SpriteCircle
   • ShapeElementList and Shape

3. Apply the decorator to the base UIWidget

   • This should cover all child widgets
   • Tests omitted since adding / removing the decorator as needed is fast and easy
   • It seems unlikely that people will be trying to copy widgets

[einarf]

👍