Cycle plotlist

[manuelbb-upb]

Description

Plots created by plotlist don't cycle properly.
Computation of the cycle_index via plot_cycle_index can now also work with PlotList objects.
We also recurse PlotList objects in scene.plots.
In diff_plotlist! we have to keep in mind that the current plotlist is not yet in scene.plots, so compute an offset.

plot_cycle_index did not look quite right with the specs, spec::PlotSpec, plot::Plot signature, but I don't know how to test it.

Seems to fix #4843 #4353 #4322

Type of change

Delete options that do not apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Added an entry in CHANGELOG.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.

[manuelbb-upb]

@SimonDanisch can you run the workflows again? On my first commits, there was some confusion because of what Revise had loaded and I was using TestEnv for the tests.
Now precompilation should work and the tests pass locally.