[GTK Config] Apply color scheme to GTK3 applications

Summary:
Move from krdb (see D28070) with some refactorings.
Depends on D28070

Test Plan:

This is how I got it to work, improvements to the process are appreciated

1. Apply D28070
2. Compile breeze-gtk and then copy $PREFIX/kde/usr/share/themes/Breeze to /usr/share/themes (replacing the existing Breeze (make a backup)). This is needed because Breeze-Gtk recently changed its color definitions names, which are used in this diff. Old Breeze-Gtk uses different ones and therefore the coloration won't work.
3. Run systemsettings and reapply Breeze theme from Application Style KCM
4. Restart kded5
5. Go to colors KCM, open it side by side with GTK3 applications (preferably Gedit NOT Nautilus or Lutris (those ones are kept open if you press cross in headerbar))
6. Change the color scheme.
7. The application must change the colorscheme

Reviewers: cblack, Plasma, ngraham

Reviewed By: cblack, Plasma, ngraham

Subscribers: ndavis, ngraham, davidre, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28072