FEATURE: add a blending optioon to the color picker

This patch adds new 'blending' functionality to the color picker (both
the dedicated tool and the ctrl-activated one) along with the associated
"blend" slider to the color picker's tool options GUI.

Patch by Emmet & Eoin O'Neill <emmetoneill.pdx@gmail.com> Thanks!

CCMAIL:emmetoneill.pdx@gmail.com
CCMAIL:kimageshop@kde.org
Differential Revision: https://phabricator.kde.org/D11895