Add lockfile to prevent duplicate instances

[wiggleforlife]

Prevents two instances from being open at the same time.

Application.main() seems like the best location for this as nothing has started yet, however this means (correct me if I'm wrong), Log4J cannot be used should another instance be detected. However, I don't think logging would be needed for something like that, so I don't think that proves an issue.

I tested this on Windows 10 and it did output to my ~/ directory, but I haven't tested on OS X or Linux. Should work, in theory.