Feature: implement semantic semantic legend entries

[cvanelteren]

With #570 and #571 the intention is to allow for easier semantic handling of legend entries. This issue holds a reminder to move towards a semantic handler. For inspiration we can checkout https://legendkit.readthedocs.io/en/latest/ (thanks @gepcel)

A must would be

• cat_legend(...): builds categorical legend entries from discrete labels.
  Example intent: category -> color/marker mapping (["A","B","C"] with palette), returns handles/labels ready for ax.legend(...).
• size_legend(...): builds a size scale legend for marker area/size encodings.
  Example intent: numeric size levels ([10, 50, 200]) rendered as different marker sizes with formatted labels.
• num_legend(...): builds a numeric-color legend from continuous values.
  Example intent: sampled bins/levels from a colormap (vmin, vmax, cmap) to show gradient steps as legend swatches when a full colorbar is not desired.

Nice to have is combined colorbar with legend in one box.