Add Support for Reading\Writing Save Metadata about save-game-copier

[slinga-homebrew]

I'm against doing this ticket for the following reasons:

1. SGC is feature complete as is. Not saving\restoring the metadata is equivalent to what the Sega Saturn Netlink does. So SGC saves are guaranteed to be as compatible as the Netlink
2. There's something nice about seeing the MD5 sums of the saves on screen. Using a metadata structure will change.
3. If I go to a metadata structure what filename do I show the user? The filename on the CD? Read the metadata and display the save name? What happens if there are multiple files on the CD that represent the same internal save filename? What happens when a user copies a Saturn save? How do I name the file? Do I overwrite an existing save? Or create a new one.

For these reasons I feel like this will be a won't fix.