Revert "Reverse my previous revert, and eliminate the attempted installation of the other Gtk 3 theme versions"

This reverts commit 19ce2441. It breaks
completely installation of themes: the wrong theme is installed (in case
of GTK+-3.20, the 3.18 version is installed) and this causes issues and
major visual artifacts.

Detected by openQA.

CCBUG: 388018
CCMAIL: bcooksley@kde.org