[Various KCMs] Notify about changes in GTK related settings

Summary:
Also save colors before saving colorScheme name, so that we can
correctly use heuristic in GTK Config, after the moment we know, that
color scheme has changed.

Test Plan: Check if the order of saving colors and saving color scheme name does not break anything.

Reviewers: Plasma, ngraham, ervin, bport, meven

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27956