Add KCM for adjusting QtQuick Rendering in Plasma

Summary:
Does what you imagine it would.
It's akin to KWin's compositor settings, designed to be hidden away and
not used by 99%.

Test Plan:
Set some values
Looked at config.
Unset set values

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: abetts, mart, graesslin, alexeymin, sebas, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7934