Setup automatic colorbars for volumeslices

[ffevotte]

Description

Fixes #3251

This PR introduces a specialized extract_colormap(plot::Combined{volumeslices}) method, in an attempt to fix #3251.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• Added an entry in NEWS.md (for new features and breaking changes)
• Added or changed relevant sections in the documentation
• Added unit tests for new algorithms, conversion methods, etc.
• Added reference image tests for new plotting functions, recipes, visual options, etc.

I'm not sure how to test this. I added

• a call to volumeslices+Colorbar in ReferenceTests/src/tests/figures_and_makielayout.jl, taking contour as a model;
• a small check in test/makielayout.jl, this time taking barplot as a model.

Is that enough?

[ffevotte]

Tests seem to fail because the Colorbar for recipes.png test image does not match its reference any more. This is expected since I added a new colorbar to the image, but I'm not sure how to update the reference image accordingly...

[SimonDanisch]

I updated the reference images!

[SimonDanisch]

Thank you!