When a session terminates, we should clean up a few things:

• pid file (on client)
  
• password file (both) - can also remove this one when the client is no longer actor
  
• icon file (both)
  
• session file - eventually? (both)
  
• ...

If the account has an SSH key defined, we can just add it to the list of authorized keys on the server rather than storing the password.
shifter_add_key SHIFTER_KEY, SSH_KEY
This is the public key, so we need the UI widgets to point to the public key as well as private key. Windows keys generated by puttygen.exe must be converted on the server via "ssh-keygen -i"

At the moment, xpra supports this on the server: the xpra server stays alive even when shifter_server is stopped.
Ideally, we want to keep all the sessions running and just re-load the session files when the server re-starts. The problem comes from the fact that NX writes to stdout and we watch this output to get the current session state, we will need to redirect to a file instead and watch that (and maybe also start it with 'nohup'?)