Extend keys and screens documentation

Add chapters `Lazy functions` and `Special keys` to the `keys.rst` and `Third-party bars` to `screens.rst`.
Do you mean the whole third-party bars idea? Of course when we have the most important wigets implemented, the third-party bar is not needed. I should probably write this in the docs. But I can imagine users, cause I am one of them, which comes from another WMs and already have configured some bar. I am pretty sure I will reimplement my script+dzen2 to Qtile's panel configuration, but why would I have to do it right now? :-)