While we may, in some distant future, move to properties everywhere,
right now providing getters and setters allows for uniform code for
touching Artist properties.

• I have a real use case in mplcursors for this feature :) (in the sense that it avoids some more hardcoding that anncoords are "special" as they are directly assigned instead of going through a setter).
• I did not add getters and setters for xyann because they already have get_position/set_position.
• In Annotation refactor #2351 @tacaswell renamed what was before the textcoords attribute to anncoords, but the keyword argument to the Annotation constructor was not changed (so it's still textcoords). I'd suggest renaming the constructor argument for consistency...

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way