Track MD5s per achievement

[Jamiras]

When an achievement is triggered, include the MD5 in the submission (toolkit change). This could be tracked on the backend and used to generate a list of completely compatible MD5s and a list of mostly-compatible MD5s for a game. It would also provide a way to see which MD5s are least commonly used, allowing for easy identification of candidates for unlinking.

See also #1

For simplicity, recommend counting unlocked achievements per MD5, rather than tracking individual unlocks.

• Counter should not be incremented if user has already unlocked the achievement (either in softcore more, or due to reactivating it manually).
• Counters should be reset if achievement is modified to reset compatibility.

[meleu]

Just a heads-up:
In this RAIntegration's PR @Jamiras makes the game hash be included in the unlock request (as m parameter). Currently the server simply ignores it, but later it can be used for the feature requested here.

It would be cool to have the same behavior in RetroArch.