Improve SpecApi performance (direct plotlist usage)

[ffreyer]

Description

In reference to #5717

This adds a specialized constructor for PlotList as well as connect_plot!(scenelike, ::PlotList) to remove all the plot setup that plotlist does not make use of. I.e. the whole argument conversion pipeline, default attributes, theming, cycling, float32converts, etc. In my local benchmarks this cuts down

@benchmark plotlist!(scene, PlotSpec[]) setup = (scene = Scene())

from around 15µs min to about 9µs min (mean ~19 to ~10)

This also makes changes to how convert_argument methods producing plotspecs are processed. In theory that should also speed things up, in practice it seems to only be marginally faster.

TODO:

• check BlockSpec and GridLayoutSpec for similar improvements
• see what tests say

As a side node, I noticed that plotlist! kind of but not really inherits attributes down to the plot spec plots. Specifically, it forwards what's in plot.kw, i.e. the initially given attributes. These attributes overwrite whatever the Spec sets and will always remain at their initial state. This seems wrong to me and I'd expect either:

• plotlist passed no attributes whatsoever to spec plots
• plotlist attributes act as defaults for spec plots (i.e. spec plots overwrite) and:
  • If plotlist attributes update, the next construction of spec plots should read the updated attributes
  • If plotlist attributes update and are used by a spec plot, the spec plot also updates

Type of change

• performance improvements (non breaking?)

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.