Extend keys and screens documentation

Add chapters `Lazy functions` and `Special keys` to the `keys.rst` and `Third-party bars` to `screens.rst`.
We both had right. It didn't work for me when I have tried it, but it work as you said. The problem is, that I forgot the `-dock` parameter for dzen2.

So, what about the Third-party bars? Just remove the whole section or rewrite it in the way, that Qtile's bar can do awesome things and there shouldn't be a need for third-party bar, but when it comes to, Qtile can handle them?