roster-window: unprepare the individual manager before destroying the roster
roster-window: unprepare the individual manager before destroying the roster

https://bugzilla.gnome.org/show_bug.cgi?id=660128

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