Portable Launcher (plus a whole slew of bugs fixes for problems I found that I suspect aren't on the tracker)

Resolves #6846 (closed)

It's late, so I won't repeat myself by going into detail about all the things I changed or fixed. It's definitely worth looking at this commit-by-commit as the changes should make sense on their own (and the comment messages explain what happened), but there are so many bug fixes that touch the same code that reviewing the amalgamation won't make much sense. It would have been possible to turn this into a stream of MRs, but they'd nearly all depend on their predecessor, so it'd have just taken ages to get this done.