Summary:
For a consistent UX use the same component for config UI elements
showing a currently selected color and allowing to trigger a color picker to
select another color.

Test Plan:
All buttons adapt to color changes as well as trigger the color dialog if
activated and take over any chosen color.

Reviewers: #plasma, #vdg, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12318