Port QRegExp to QRegularExpression
Summary: Require Qt 5.12 for QRegularExpression::achoredPattern()
Test Plan: The code builds and unit tests pass.
Reviewers: Baloo, meven, bruns, astippich, mlaurent, apol, dfaure
Reviewed By: meven, dfaure
Subscribers: kde-frameworks-devel
Tags: Frameworks, Baloo
Maniphest Tasks: T12279
Differential Revision: https://phabricator.kde.org/D26099