room: Add method to send "identify" messages

The method is a thin wrapper around tp_text_channel_send_message_async()
which doesn't add much convenience, but it allows us to differentiate
between the "identify" messages sent by the user and the ones we send
automatically, so that we don't ask the user whether to save the saved
password again ...

https://bugzilla.gnome.org/show_bug.cgi?id=709982