muc-channel: add closed attribute to avoid to fire the Closed signal more than once
muc-channel: add closed attribute to avoid to fire the Closed signal more than once

freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut