Use better naming

[Edvinas01]

There is a set of methods/classes that I think are named poorly. For example:

• IFixedUpdateListener.OnFixedUpdated - doesn't read well (compare it to `IUpdateListener.OnUpdated).
• GameManager.OnBeforeInitializeSystems - doesn't communicate well enough that this is the place to initialize game manager.
• GameManager.OnAfterInitializeSystems - similar the one above.

There are more things that could be named better, though these were on top of my head for now...

So:

• Propose and use better naming.
• Update documentation.