UIDummy clarity improvements

[pushfoo]

Changes

tl;dr minor clarity improvements discovered during work on a follow-up to #1750

1. Clean up UIDummy top-level docstring
2. Fix a PyCharm linter highlight by adding UIDummy to an __all__ list
3. Use Color.random(a=255) instead of inlined RGB tuple generation

How to test

1. python -m arcade.gui.examples.box_layout
2. Make sure UIDummy instances still change colors when clicked
3. Make sure UIDummy instances still print their rect values to the terminal when clicked

Follow-ups

I'll be submitting another PR containing docstring fixes for the GUI examples later.