Allow to have multiple savestates
Migrated from https://bugzilla.gnome.org/show_bug.cgi?id=780733
Currently we support only single saved state, it is used to resume after we quit the game. We should add support for multiple saved states.
Adrien Plazas described the following complications on IRC:
When you stop playing we save two type of RAMs: the "save" RAM, which is your permanent save, the same as in your cartridges; and the full RAM, which contains the save RAM too.
When you select a game, we always restore the save RAM and let the rest of the RAM untouched so it's as if you were starting your actual console; but we also allow you to restore the full RAM so you take back EXACTLY where you left
So if you restore an old state (full RAM) it will also restore the save RAM because it's part of the full one, and when saving we will save the save RAM at this old state, and we don't want that to be destructive