start processes using twisted: use ProcessProtocol

[Antoine Martin]

When starting a process and watching its output, we shouldn't do this in a separate thread but instead integrate the code with twisted main-loop (gtk/glib's actually) using ProcessProtocol?.

Also, when waiting for a process by pid (one we may not have launched ourselves), we should use gobject.child_watch_add(pid) which will also integrate with the main loop.