channel_close_cb: complete in an idle
channel_close_cb: complete in an idle

tp_cli callbacks don't guarantee not to call the callback re-entrantly:
if the interface is absent, or the proxy has been invalidated, they'll
call the callback before returning. Sad times.

Also document why channel_remove_self_cb doesn't have to do this: it's
only called after preparing the GROUP feature, which does guarantee to
be idle.

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib