Deprecate QuadContourSet.allsegs, .allkinds, .tcolors, .tlinewidths.

[anntzer]

Directly construct the relevant path objects and the collections with the right properties instead of carrying around another copy of the information in raw array form. By having fewer intermediate objects lying around for a long time (as attributes of the QuadContourSet) this makes the logic easier to follow. This also prepares for #25128, but I'd say the change is worthwhile even if we eventually decide not to go that route.

PR Summary

PR Checklist

Documentation and Tests

• Has pytest style unit tests (and pytest passes)
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• New plotting related features are documented with examples.

Release Notes

• New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst

[tacaswell]

I strongly suspect that these are widely used by people who want to extract contour computations from Matplotlib. We may have to settle for converting these to just properties (or a longer than normal deprecation cycle).

[anntzer]

At least tcolors and tlinewidths should be safe to deprecate. I'd say allsegs and allkinds are better provided by contourpy nowadays, but your remark is well taken and we can at least make the deprecation pending (or even keep them around, if you think that's important, I don't feel very strongly about this).

[jklymak]

@ianthomas23 did you have time to comment on this?

[ianthomas23]

Yes, I am 👍 on this. As Tom says the allsegs and allkinds might be being used by quite a few people, but they shouldn't be from now on and can be redirected to contourpy.

[QuLogic]

The other deprecations could probably do with a test to ensure they don't break, espectially since the pending ones may be around for a while.

[anntzer]

Sure, done.

[QuLogic]

Just need to ensure these are the right warnings.

[anntzer]

Sure, done.