Add some const &

Summary:
Won't make things go much faster since everything that was
being passed by value is refcounted but still const & is a bit faster
than refcounting

For shared pointers instead of adding const & we move them into the
destination variable saving some cpu usage but at the same time making
clear the pointer is being stored by not being const &

Reviewers: zzag

Reviewed By: zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25022