This does 2 things:
- Change the .Plist to associate 0 A.D. with .pyromod files (as owner, so the default choice), and .zip (as alternate, so not the default, but can be selecte with 'open with').
- Make 0 A.D. actually load mods opened like that on MacOS. The files aren't actually opened with command line arguments, but via Cocoa stuff, which SDL2 handles via the 'DropEvent' event. This means we need a new path to load mods. I don't think it's too bad, but this whole "mod enabling/installing" thing needs some serious cleanup.