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