[kcm-colors] Export colorscheme to GTK color definitions

Summary:
This exports the active colorscheme to ~/.config/gtk-3.0/colors.css and sets up ~/.config/gtk-3.0/gtk.css to read from it.
This will do nothing visibly without changes to Breeze GTK made in D22877

FEATURE: 356006
FIXED-IN: 5.17.0

Test Plan: See that ~/.config/gtk-3.0/colors.css is created, colorscheme is dumped correctly, and that gtk.css links to it.

Reviewers: #plasma, ngraham, broulik

Reviewed By: #plasma, ngraham, broulik

Subscribers: ahiemstra, ngraham, davidre, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22844