[KFileWidgets] Port QRegExp to QRegularExpression

Summary:
Optimise the code in checkPreviewInternal(): use const where possible,
and use range-for instead of iterator-based loops.

Test Plan: make && ctest

Reviewers: dfaure, apol

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26339