Support ssh-tunneling for connections

Support ssh-tunneling for connections by supplying the path to the local ssh key, ssh username and host to tunnel to. This way, users won't have to set up ssh tunnels separately when they wish to connect to machines on protected networks.

Preferably, this should pick a random local port and forward connections only throughout the lifetime of the MytermConnection. Once the connection is closed, the underlying tunnel should be closed, too.