Complete openpyxl title type annotations

[Avasam]

Extracted and improved from #9511

• Completed and updated all title related type annotations I could find
• Use a _Decodable protocol instead of bytes for _WorkbookChild.title's setter. The implementation explicitly checks for a decode attribute for non-string values instead of expecting a bytes (which relates to Track review of bytes types #9006)
• Didn't include what's already done in openpyxl: Class attributes set explicitly as another class' #10549

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

graphql-core (https://github.com/graphql-python/graphql-core): typechecking got 1.26x slower (44.4s -> 55.9s)
(Performance measurements are based on a single noisy sample)

[srittau]

I did not compare to implementation, but looks plausible to me.