Associate .zip and .pyromod files with 0 A.D. on MacOS
- 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 will appaer in the 'open with' dropdown).
- 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 ought also work on windows and linux.
Closes #5357
Differential Revision: https://code.wildfiregames.com/D3190