Add hook to XComGameInfo.StartMatch for mods to perform custom setup at the start of a game session

[robojumper]

Game session in unreal context -- HeadquartersGame, TacticalGame, ...

Mods can then perform their startup logic: Register observers and such before any game states are added. This should work for everything: game load, new game start, level transition...

[bountygiver]

also probably a pre tactical end cleanup hook, currently there's only onpostmission which is VERY late so people have been hooking on mission summary UI instead.