BaseControl: Support accessible, flexible layouts

[mirka]

What problem does this address?

We are running into issues where we want to tweak the layout of the help text, or reuse just the styles of the help text. Currently, it requires CSS overrides of BaseControl internals, or copy-pasting styles.

What is your proposed solution?

For example, export a BaseControl.VisualHelp like we do with BaseControl.VisualLabel. This must be accompanied by clear accessibility guidance in the docs. This is just a potential solution, there may be better solutions.