We now listen to the 'connection_unexpectedly_disconnected'
signal, which is the callback for unexpected disconnections.
 'connection_unexpectedly_disconnected' is handled in
Connection.disconnect_cb and it now calls
Connection.disconnect to formally close the connection.

TkUI should change the bg color of a disconnected
conversation.  It should turn one color if your connection
is disconnected and a different color the connection of your
friend is disconnected.

TkAE already listens to connection_disconnected, so this
just needs to be tested.  It should start listening to the
new signal too.