shares StyleSetProps instances between StyleSet instances with identical paths which improves performance for sibling items.
shares properties (PropertyMap) between StyleSetProps instances that don't have their own properties which improves performance for shared ancestor trees.
This PR
Based on https://github.com/Ableton/aqt-stylesheets/pull/32.
@gck-ableton: please review! Thanks! @mak-ableton: FYI