Allow to change "ssh" argument from GUI

[Daniel Hahler]

Given the "New connection" dialog, you can only select between "SSH" and direct.

I've filed an issue with xpra (​http://code.google.com/p/partiwm/issues/detail?id=28) to connect through an inner host via SSH.

The solution I've found was to use "--ssh 'outer ssh inner'" to make this work.

So, the New Connection dialog (but any other connection editing dialogs also), should allow to specify the ssh command to be used.

Alternatively/Additionally?, as mentioned in the xpra issue, "outer:inner" could get recognized and handled as an array of hosts in the first place.