Optimise setting up the palette

Summary:
For every setBrush/setColor we're initialising a new QBrush instance that
allocates some space in memory. At the moment we were creating and destroying
QBrush instances like crazy, this check aloen saves 23K calls only starting
Discover.

Test Plan: Tried with Discover, KAlgebra and Kirgami Gallery

Reviewers: mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D14387