Add type annotations, minor cleanups

[NMertsch]

Describe your changes in detail

• diff Unify how App/Window/Widget IDs are created and documented, and ensures that the ID is always a string.
• diff Add type annotations to user-facing classes
• Refs Add type annotations for callbacks #2042 by adding typing.Protocols for callback functions
• Fix a few typos (a/an, missing comma, ...)

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

[freakboy3742]

Looks like a good cleanup; there's a minor tweak that can be made to the docstrings. The doctoring on app.py looks like it was historically wrong because it didn't have a type annotation.

[freakboy3742]

FYI: The Linux CI failure will be fixed by #2052. The Android CI failure will be fixed by beeware/briefcase#1380.

[freakboy3742]

Thanks for these improvements. I've made a couple of minor tweaks to wording and naming of the new protocols, but otherwise this looks great - and will make the task of rolling out the analogous changes to other widget classes a lot easier (as we now have a template to follow).

Out of interest - did you log the bug with Sphinx around the :rtype: spelling error? I found a little more detail on this one - if you have any :param: or :returns: declaration, the "spelling error" goes away (e.g., adding a :param widget: declaration to the OnPress handler fixes the spelling error with the None return type.

[NMertsch]

Looks great, thank you!

@freakboy3742 yes, I reported it here: sphinx-contrib/spelling#216

It seems to be caused by the autoprocol plugin. Won't have time to investigate it in the next days, though.