Add some sort of symlink management for indirect backups

[mtkennerly]

GSM has a "Steam Spreader" function for moving/symlinking game installs, as well as a "Link & Sync" function for doing the same just for save data. It may be worth adding similar functionality here for users coming from GSM, since it provides a way to redirect game saves to a cloud storage folder or alternative drive, thus acting as another form of backup. This would require some care to ensure it works well cross-platform and that it won't break anything (e.g., if folders/files are affected that shouldn't be or if it could interfere with the operation of Steam/etc).