share common values for both Breeze and Breeze-dark GTK themes

Summary:
BUG: 396091

As described in bug #396091, the Breeze-dark theme is often neglected in updates. This patch fixes #396091 and some additional, related inconsistencies, enables sharing the basic stuff like shape, size, style, etc. of components in a common single css file while keeping the colour definitions separated from that in different files for both the light and dark theme.
This allows easier changes, easier maintenance because of less LOC, less duplicate code as well as easier extensibility for potential additional colour schemes like [[ https://hig.kde.org/style/color/light.html | light ]] or [[ https://hig.kde.org/style/color/high.html | high-contrast ]].

I only did this for GTK 3.20 for now. If you like the effort, it could (and maybe should) get extended to other versions.

Further potential steps in the same direction of saving code would be going back to SASS, which is used by many other popular GTK themes and was used by Breeze-gtk as well in the past. The only downside of that is that the SASS source files would have to be "compiled" to CSS prior to packaging.

Test Plan: I don't know of any automated test suites comparing UI components. I have already posted screenshots in the original bug report and can post additional ones of the new state for comparison. Apart from that, my manual sparse testing makes me feel the dark GTK theme looks and feels much more consistent to Breeze now.

Reviewers: jackg, #breeze, #plasma, ngraham

Reviewed By: #breeze, ngraham

Subscribers: grmat, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16365