High score-aware Autosaves / Continues

[cxong]

Autosaves persists some player state such as guns and ammo, but the persisted state should be shared between the high score system, so that game continues work well with high scores.

That is, suppose the player plays mission 1 in one session, then continues and plays mission 2 in the next session. Normally this would result in two high score entries. However if we produce a play id (i.e. timestamp of when the campaign was started) and associate it with the game continue, we can reuse the first high score entry when starting the next play session, and overwrite it when the high scores are saved. For this to work, all high score relevant fields need to be saved, such as number of kills, mission time and so on.

To make this work well, we need to:

• Combine the autosave and high score files
• Extend the high score entries so that they are keyed by the play id
• Optionally, add a submenu to the continue menu so that we can have multiple play sessions
• Once a campaign is complete, that session will need to be disabled, so that the player can't repeatedly replay the last mission to scum for a high score
• Historical high scores will be associated with disabled play sessions