Fix Clazy performance issues, const &
Summary: * Fix a lot of missing const & or &, avoiding copying mostly QString.
Test Plan: ctest
Reviewers: Frameworks, dfaure, kossebau
Reviewed By: kossebau
Subscribers: ahmadsamir, anthonyfieroni, kossebau, kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D25039