macOS: Change default savegame location for macOS

[Thunderforge]

In response to #11427 involving all sorts of issues if users deny ScummVM permission to access the Documents folder, I would like to change the savegame location from ~/Documents/ScummVM Savegames to ~/Library/Application Support/ScummVM/Savegames, which is the recommended location according to ​Apple's developer guidelines:

Contains all app-specific data and support files. These are the files that your app creates and manages on behalf of the user and can include files that contain user data.

This will prevent new users from experiencing the issue since ScummVM will not need access to Documents, meaning users will not be prompted. (No special permissions are needed to access Application Support).

A migration like this is similar to one performed on Windows around 2012 in which users were directed to ​run this script. This was a manual process and ideally we would want to come up with something more elegant and automated, such as ScummVM detecting if a migration needs to happen for existing users while preventing the need to check the Documents folder for any existing users.

[somaen]

Sounds good, after Documents started defaulting to iCloud there's even some oddities if permissions ARE given, so would be good to move the location.

[sev-]

Cross-referencing #11427