Seamless

- [x] make it seamless (i.e. without a desktop; the Xpra windows are then integrated into the client's desktop as if they were stand-alone windows)
- [x] use a patched Xdummy to allow virtual screens larger than 2048x2048 (see http://xpra.org/trac/wiki/Xdummy)
- [x] avoid generating a password by using a private/public key pair
- [x] start the Xpra desktop right away, not only the `sshd`
- [x] provide a convenient shell script to connect to the running container
- [ ] fix `gnome-terminal` not rendering properly (might be an issue with my 0.9.8 client)