Save a machine on macOS

#1
- The default folder for saved machines (user data, screenshot, etc.) is on the app bundle on macOS, it should be on ~/Library/Application Support/[Besiege or unity.spiderlinggames.besiege]

- An app bundle on macOS doesn't contain user files, If the bundle is signed or the app is reinstalled, you lose all your work.
- The game permit to save a machine without a name, result of a machine saved in an invisible file (.bsg)
- I don't know if there is a way to rename the machine.
- I don't know if it is possible to change default machines folder path.
 
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