macOS: self-moving to avoid issues with the downloads folder

[NuclearMonster]

Currently, if a user downloads a .app bundle of ioquake3 and runs it from their /Users//Downloads folder the .app will be unable in some cases to interact with the filesystem. As a workaround, the .app may prompt and move itself to another folder. See these examples for explanations of how this happened and the recommended workarounds:
https://github.com/potionfactory/LetsMove/
https://github.com/iglance/AppMover

[ensiform]

ET: Legacy has the fix for this in their repo. But it's under v3 terms, however you could ask them for usage and relicense that snippet under 2+ for ioquake3.

[NuclearMonster]

@tomkidd any thoughts on this issue?