Add RetroAchievements

[MaddTheSane]

Many people have requested RetroAchievements be added to OpenEmu. I don't think that's impossible.

• Hook up a UI for logging-in to RetroAchievements.
• Discuss what the achievement banners should look like.
• Allow hooking up Cores in a way that RetroAchievements can use them. Perhaps change the system plug-in to accomplish this?

[wescopeland]

Hi @MaddTheSane, I'm an admin over at RetroAchievements. Happy to see this could potentially be getting some traction.

Our rc_client integration guide may be helpful: https://github.com/RetroAchievements/rcheevos/wiki/rc_client-integration

Also, I'm not sure if you're in our Discord, but if you are interested in pursuing this, we have a special private channel for integration support that I could have you added to.