Update type annotations for audited widgets.

[freakboy3742]

At some point during the audit, we decided to introduce type annotations as part of documentation.

This PR retrospectively applies type annotations (including adding from __future__ import annotations so we can benefit from the most recent set of typing conventions).

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[mhsmith]

I wasn't sure about arguments that accept int or float being annotated as float, but it turns out this has a special case in PEP 484:

Rather than requiring that users write import numbers and then use numbers.Float etc., this PEP proposes a straightforward shortcut that is almost as effective: when an argument is annotated as having type float, an argument of type int is acceptable

And this is implemented by mypy.