Maintain artist addition order in Axes.mouseover_set.

[anntzer]

PR Summary

Fixes #11284, see discussion there.
Perhaps deprecate mouseover_set? Edit: yes.
Perhaps a test would be nice.

PR Checklist

• Has Pytest style unit tests
• Code is PEP 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

[tacaswell]

I have a slight concern that this is in a hot-loop on mouse-move so a slight preference for deprecating mouseover_set and replacing it with a (private?) list. At least we should use the private version of this set in backend bases.

[anntzer]

Deprecated the public version, used the private version internally. As _OrderedSet.__iter__ just calls iter(self._od) (rather than yield from self._od), that will instantiate the native iterator on the OrderedDict instead of requiring going through an additional Python frame at every iteration.

Maintaining a list is also possible but it's just a very minor pain to reimplement add to check whether the item was already present or not, as well as whether discard should ignore missing elements (as set.discard does, but list.remove doesn't) (yes, that shouldn't happen as long as the state is internally consistent). It's not hard but feel free to write a PR if you want :p

[tacaswell]

@anntzer rebased and force-pushed.

[tacaswell]

Anyone (including @anntzer ) can merge this if CI passes.

[anntzer]

Self-merging per #11287 (review).