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! We're letting users launch KSP! In which case, two requests:

- If it doesn't already allow it, provide a way to specify the whole command line, not just options for KSP itself. I run KSP with `optirun ./KSP.x86_64`, which means I get the 64-bit version (which works great under Linux) and that it uses my nvidia graphics card (as opposed to my low-powered intel one).

- Can we pop extra features like this on separate branches in the future? This PR is pretty big, and it keeps getting bigger. That makes it really hard to review!

I'm cool with PRs being on branches that stemmed form other branches (although happier still if they all stem from master), as this still means I can review them in much more bite-sized chunks.