refactor: fix various compiler warnings

Summary:
Fixes pessimizing move, implicit copy assignment operator, copies
in range for loops, deprecated Qt methods, missing final keyword
and unused lambda capture.

Built with Clang 10

Reviewers: Plasma, romangg

Reviewed By: Plasma, romangg

Subscribers: romangg, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27626