Set XDG_CURRENT_DESKTOP if unset

XDG_CURRENT_DESKTOP is used by glib >= 2.42 at g_desktop_app_info* functions

Original GNOME commit: https://git.gnome.org/browse/gnome-session/commit/?id=1db085ce7e5197b28ab24a3ec54d888efe98fcb4
Closes https://github.com/mate-desktop/mate-session-manager/pull/62