Add active, link, visited, negative, neutral and positive background colors

Summary: Buttons colored with special text role colors as backgrounds don't always look nice with black text on light themes. They should have been using background role colors for the background, but the background role colors they needed weren't available. This patch adds the missing background role colors.

Reviewers: Plasma, Kirigami, mart

Reviewed By: Plasma, Kirigami, mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26745