Properly look for fallback themes

Summary:
At the moment we always look at the first fallback and load the original
theme again, rendering moot the fallback look-up.

Reviewers: Plasma, davidedmundson, hein

Reviewed By: Plasma, davidedmundson, hein

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D15970