Port QRegExp to QRegularExpression

Summary:
With QRegularExpression \x escape sequences that use more than two
hex digists have be in the form \x{nnnn}.

Test Plan:
make && ctest
All unit tests pass except kcontacts-addresstest, but it fails with the
same exact error on master too, so it's not any worse.

Reviewers: mlaurent, vkrause, apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T12279

Differential Revision: https://phabricator.kde.org/D26145