updates: Let gnome-software handle update checking

Starting with 3.12, gnome-software has its own service for checking new
updates and notifying the user. To avoid stepping on its toes, this
commit completely disables the g-s-d update polling when gnome-software
is installed.

https://bugzilla.gnome.org/show_bug.cgi?id=727354
[l10n] Update Catalan translation