Port QRegExp to QRegularExpression

Summary:
Port QRegExp::exactMatch() by using QRegularExpression::anchoredPattern()
to match the entire subject string.

Remove filenameOnly(), it's been broken for a long time (QStringLiteral("[/\\]")
is not a valid QRegExp pattern); besides it's not needed as QFileInfo::fileName()
is used to get the filename.

Test Plan: make && ctest

Reviewers: Frameworks, dfaure, ervin, apol

Reviewed By: dfaure, ervin

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26177