* add VARIANT and VARIANT_DESC to the list of known vars in the drawing
sheet editor.
* cache variant and variant description in respective objects.
* expand the respective text variables when building the drawing sheet.
* keep the cached variant and its description up-to-date when selecting
a variant in the GUI.
* pass variant and its description to the plotters.
Fixes https://gitlab.com/kicad/code/kicad/-/issues/23430