roster-window: re-hook the show-groups gsettings key
roster-window: re-hook the show-groups gsettings key

It has been accidentally removed during the recent settings clean up.

GNOME/telepathy-account-widgets,GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets