[widget audit] toga.ActivityIndicator

[freakboy3742]

A testing and documentation audit of the toga.ActivityIndicator widget.

Includes the content from #1799.

Audit checklist

• Core tests ported to pytest
• Core tests at 100% coverage
• Core docstrings complete, and in Sphinx format
• Documentation complete and consistently formatted
• cocoa backend at 100% coverage
• winforms backend at 100% coverage
• gtk backend at 100% coverage
• iOS backend at 100% coverage
• android backend at 100% coverage
• Widget support table updated

[freakboy3742]

This is 100% coverage on the platforms that have currently implemented ActivityIndicator. It's not a complex widget to implement - winforms is the only complication because it doesn't have a native spinner widget (AFAICT).

The question becomes whether we treat the goal of this audit as "test all the widgets/features currently implemented", or "complete the implementations". My inclination for now is to do the latter, if only because having tasks like this on the backlog makes finding sprint tasks much easier, and we can get a much better impression of where we are at before getting bogged down in actually fixing them.