Fix warnings

Summary:
Remove unused variables and members.
We were often storing the q of classes without ever needing them. I'd
just do it when it's useful. We do it in the private class so it can
always be added, removes boilerplate code.

Don't use std::move when returning temporary QImage. The compiler is
smart enough to know to do it but produces the warning "moving a
temporary object prevents copy elision".

Remove unused lambda captured variables.

Test Plan: Getting much cleaner build logs

Reviewers: #frameworks, #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D28442