Split from #168 - see there for details.

What we want is the ability for the user connected to the session to decide what to do with requests (within that session) for opening new files and URLs. Give them 3 options:

• open them as normal (application used will be part of the same session as the one that launched it) - current behaviour
• open them in a new session (create a new session and attach it automatically)
• open them on the client (only valid for URLs, files would have to be copied over first which is a lot more tricky!)

Some of the exception handling is broken (references non-existent attributes or not using all arguments in string formatting). Attached patch fixes both I found.

0.12.8 fixed a number of bugs in this area however there can still be a case where we end up not being able to connect to a server. Either because of a race which makes us start two copies? Or something else?