Adjust OffsetBox.get_window_extent to use get_extent (which is typically redefined in subclasses) instead of get_extent_offsets (which is not), and make it check whether the current class' get_offset takes any arguments or not (which is not consistent, but fixing that requires some backcompat work).

Also set the offset-getter of AnchoredOffsetbox as a fixed method on the class, rather than dynamicaly setting in that class' get_extent_offset.

These changes then let us inherit the implementation of OffsetBox.get_window_extent across most subclasses instead of having to copy-paste it again and again.

This mostly replaces #21699 without touching the more complex question of changing/harmonizing the signature of get_offset.

PR Summary

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• 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).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).