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 
Uh oh, this branch needs a comment because I have no idea what it's doing. It looks like it creating a file and then deleting it again, but I have no idea why we'd do that. And it happens if not `neverOverwrite`, which is the same as saying we do it overwrite is true.... But we're not overwriting files here, we're just creating them and deleting them again. So honestly, I'm confused here.

- [ ] Make pjf less confused.