Let's use the same principle with DESKTOP tag as we use with
LANGUAGE tag. Where there are any GNOME-only (or KDE-only) needles, tag
them with `DESKTOP-gnome` and delete then when different `DESKTOP` value
is specified. This will help with `QA:Testcase_Package_Sets_KDE_Package_Install`
testcase - we will be able to use almost the same code and check that KDE
got installed properly.
Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D670
