Sort the themes in decoration KCM

Summary:
Even though we're using a QSortFilterProxy model, by default it doesn't
actually sort anything until instructed to.

This patch turns sorting on.

Test Plan:
Opened the KCM
Looked at it

Reviewers: #plasma, mart

Reviewed By: mart

Subscribers: plasma-devel, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D5407