Multi-instance support

Only works in the GUI for now, doesn't break  the command-line client but the new options are not implemented.

Related to #29 
Oh noes! I'm confused again! Why do we have a command line dialog? Isn't it too late to be reading command line options after the GUI has already started? Do you think my confusion could be cured with some docs? ;)

- [ ] Make @pjf less confused. ;)