Allow local native AI to pick its start position.

Move AI init from CGame::StartPlaying to CGame::LoadGame.

This change will affect Native AI initialization (and Lua's also). ZK tests were good (CAI, Chickens, Shard, CppTestAI, own LocalTestAI with sendStartPosition at EVENT_INIT).

1 caveat for native AIs: they should use
sendStartPosition(false, position);
instead of
sendStartPosition(true, position);
Somehow "true" will signal that the player (not AI only) is ready.