The default savepath on macOS used to be in the user Document folder.
However ScummVM requires special permission to access that folder,
and if the user denied it, this caused various issues.
This implement ticket #11428.
This commit also contains migration code that that old configs that
used the default path are set explicitly to use the old default path.
So the change of savepath will only be for new users.
Apple's desktop operating system was formerly called "Mac OS X" and "OS X", but since 2016 it has been called "macOS" (starting with version 10.12).
Changing across all comments and documentation to use this current terminology, except in cases where the historical versions are explicitly referenced. No code changes are made; we should consider changing those in future PRs.
