Go back to SCSS.

This patch tries to go back to SCSS sources with backporting all the
fixes made in Breeze-gtk recently.

As a base, I used this repository:
https://github.com/dirruk1/gnome-breeze

But it's outdated, so I walked through the commit history of breeze-gtk
and ported all the changes here.

The script for building the theme was ported to Python3, and shell
scripts now use POSIX sh instead of bash.

Also I fixed some inconsistences between Qt and GTK Breeze themes. For
example, Breeze-Qt colorscheme was updated a little (in Plasma 5.12, as
far as I remember), but the GTK theme was updated only partially; now
the colors are synchronized. Both GTK-3.18 and GTK-3.20 versions work
fine for me; many things were also fixed for GTK 3.18.

Differential Revision: https://phabricator.kde.org/D17154