main: tell Qt applications to look like GNOME

If the user is logging into GNOME then QT applications run in the
session should integrate as good as possible.

According to KDE folks, the best way to achieve that is to set

QT_QPA_PLATFORMTHEME="qgnomeplatform"

This commit does that.

https://bugzilla.gnome.org/show_bug.cgi?id=762681