Revamp of AGI savegame system
|Reported by:||spookypeanut||Owned by:||sev-|
Converted AGI savegame system so that it no longer relies on Common::File. Also put all save related functions under AgiEngine, rather than being under SaveGameMgr as they were before, and removed SaveGameMgr.
Ticket imported from: #1631229. Ticket imported from: patches/701.