Accept the new VisualType in Static and Label

[davidfokkema]

This change keeps mypy happy.

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

[davidfokkema]

Thanks to @TomJGooding pointing out the solution to #5617.

[willmcgugan]

I think this is right.

That setter should probably be renamed to content. Since that's a breaking change, it probably makes sense to put it off until the next major release.

Would you mind adding a TODO for that in this PR?

[davidfokkema]

I added TODO items for the Label parameter renderable and the setter on Static.