Special saves!

All this refactoring, just to resolve #162. Specifically, it engages savegame events not at level load (except for savefile start) but on level completion, just after you've gotten all the intermission bonuses but before the intermission actually starts.

Also fixes a never-before-discovered bug where if the titlemap has LF_SAVEGAME, your save file will be overwritten upon returning to the title screen. This game is a mess of hacks, I swear...

One unintended side effect: It may actually be faster in some speedrun circumstances in mods with cutscenes to complete the map, exit back to the title screen, and reload the file. It's a common feature of optimal runs in games with cutscenes, though, and Marathon Run has a toggle for cutscenes, so I'm not particularly bothered.