Since the introduction of https://build.opensuse.org/request/show/436688, mate
is no longer able to be installed by a user without resolving a conflict, even
if the system was first installed using minimalx (let alone if any bigger DE
was chosen)

The reason for the conflict is in the new requirement of
libqt5-qtstyleplugins-platformtheme-gtk2, which in turn conflicts with the
-gtk3 variant.

The gtk3 variant is auto-triggered if Qt5 and gtk3 are both triggered for
installation on a system (which is pretty much the case in every setup
nowadays)

It would be great if there was a better way to come up here:
* Either figure out a way to not conflict, and only load one of the platform
themes (maube gtk3 by default, unless a ENVVAR specifies differently; this
would allows mate-session for example to export the variable and the
platformthemes would not have to conflict
* Have mate only recommend the platformtheme-gtk2: I realize though that this
will mean you basically never get it installed