Move entries from `game_state` to `system`?

[tomlogic]

Some of the entries in game_state seem like they're more appropriate for a new top-level category that I'll tentatively call system. Candidates for this section:

• credits: Current number of credits on the game.
• max_credits: Maximum number of credits allowed.
• volume: Current volume setting. Entry should have a min/max value
  so it's possible to represent the volume as a percentage, and to know
  the valid range for making changes.
• replay: Current score needed to achieve a replay.
• last_game: Timestamp of last game played.
• game_id: Field on WPC games identifying the machine (e.g., 50028 on Demolition Man).