Sort the themes in decoration KCM
Even though we're using a QSortFilterProxy model, by default it doesn't
actually sort anything until instructed to.
This patch turns sorting on.
Opened the KCM
Looked at it
Reviewers: Plasma, mart
Reviewed By: mart
Subscribers: plasma-devel, kwin, KWin
Differential Revision: https://phabricator.kde.org/D5407