This class aims to implement Design Token idea into FreeCAD themes. It
allows themes to use generic variables with generic values so we could
use one qss theme and change the style based on values from preference
packs.
This changes current "Display/UI" pages to "Display/Advenced" as most of
settings there were pretty technical. These settings are not removed as
it is hard to decide which one should be removed and which one should
stay and we are too close to UI freeze.
Settings that should be obviously kept were moved to new "Display/UI"
section which now includes also options from previous "Display/Theme".
Fixes: #14412
