Improve handling of the start game

[wraitii]

The way we currently start a game is fairly messy, with a lot of inits everywhere, a "ReallyStartGame" which is also called when you use Atlas (and thus don't really start a game) which is really messy.

Furthermore, we use "InitGame.js" for the last part of the initialization, but not everything (skirmish entities replacement is done in a separate function because we need to flush "Destroy" Messages for the AI).

This is tied a lot to AI initialization.

It'd be nice, if anything, to have a wiki page listing how we currently start a game, for future reference.