Async level initialization

[fornever]

Async level initialization

[ettud]

TODO[#47] is no longer available and the code has changed. We have let! level = Level.Load directory 1 2 during the game initialization. Do you suggest to make it a delayed async computation?

[ForNeVeR]

1. Anything that does network or disk IO should be asynchronous.
2. Please wait for #68, that PR introduces a lot of async activities for content loading. There's no async execution mechanism for in-game mode, but we may discuss that after the PR is merged. I imagine something similar to the LoadingLoop (https://github.com/ForNeVeR/O21/blob/cc5c046af966cb09cddbcad6986efc63b42d378b/O21.Game/Loading/LoadingLoop.fs) in the main game.

[ForNeVeR]

I think that the level initialization is already async right now.

When adding new level support, we'll load them all at once (that's just easier and shouldn't be a problem with our level count and memory requirements).