Refactor QString::contains(QRegularExpression)

Unify the code of the two overloads; fix a typo in a warning;
and use an opportunity to insert a move.

Change-Id: I85eef86d35f0d1e2da25f09d92204e32abf6d7d3
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>