Better use of Qt APIs in Plasma::Theme

Summary:
Try not to access hashes repeatedly.
QRegExp -> QRegularExpression.

Reviewers: Plasma, Frameworks, fvogt, mart

Reviewed By: Plasma, mart

Subscribers: bruns, tcanabrava, fvogt, broulik, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22147