Dedupe some box anchoring code between legend.py and offsetbox.py.

[anntzer]

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[timhoffm]

+/- optional changes.

On a more general note, it feels wrong that legend needs to explicitly handle locating. Shouldn't a legend be some subclass of an anchored box (I'm not very familiar with that part of the library, so I don't know which class exactly)?

[anntzer]

It's probably possible to make legend reuse more of the offsetbox code, but there'd need to be quite a bit of backcompat/surgery due to different attribute names, etc. so that'd be a separate project.

[QuLogic]

Did you not push the optional changes?

[anntzer]

oopsie.