overhaul RatingStar

Summary:
removes the copy & paste code in RatingStar and uses a repeater instead.
Fixes T7529 and also the recently reported bug. Rating can now be deselected on every star.
Also cleans up the size definition for the star.

Reviewers: Elisa, mgallien

Reviewed By: Elisa, mgallien

Subscribers: mgallien

Tags: Elisa

Maniphest Tasks: T7529

Differential Revision: https://phabricator.kde.org/D9185