Consistently use KQuickControls.ColorButton in the config UI

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