Add possibility to sort by rating

Summary:
Gwenview allows to rate images but there is no option to sort images by rating.

Add 'Rating' as a sorting option to Gwenview's SortedDirModel.
Add 'Rating' to Gwenview's 'Sort By' menu.
Add 'Rating' to Gwenview's 'Sorting' configuration entry.

Depends on: D13669

FEATURE: 340667

Test Plan:
- Give specific ratings to some images in Gwenview.
- Click {nav View > Sort By > Rating}.
- The images should be sorted according to their ratings.

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: muhlenpfordt, rkflx, broulik

Differential Revision: https://phabricator.kde.org/D13344