Micro optimizations

Summary:
Add some const & for things that return const & so there's no need to make a copy
Add a std::move for a thing that we pass by copy and it's just saved in another variable

Reviewers: zzag

Reviewed By: zzag

Subscribers: zzag, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24614