export local displays to a remote server via a reverse tunnel

[Antoine Martin]

Now that we can export local displays (see ​changeset and #12)

We can export these displays to a remote server by getting the server to give us a remote port and setting up a reverse tunnel to a local port on the client (reserved for nx shadow process).
When we request connection (detect local access and use a shortcut - or even warning if same display as current), get the client to start the nx shadow, then it sends the session to the actual client.