[Color Picker] Add contrast frame around colors in popup

This adds a frame around the color sections, makes it look a lot nicer and improves contrast.

Since here the frame takes only a minor portion of surface area compared to the circle in a panel,
I opted not to base it on luminosity, also for consistency's sake when there's 9 different colors in one view.

Differential Revision: https://phabricator.kde.org/D4298