[GTK3] Rename colour definitions

Summary:
Colour definitions are now suffixed with _breeze, and public API colours
are now reexported variables.

CCBUG: 417461

Test Plan:
In tandem with D27426, no visual effect should
be noticeable on Breeze GTK.

Other themes should display an improvement:

Before: {F8105241}
After: {F8105244}

Reviewers: #breeze, ngraham

Reviewed By: #breeze, ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27425