[kcm-colors] Export colorscheme to GTK color definitions
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
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
Differential Revision: https://phabricator.kde.org/D22844