Extend keys and screens documentation

Add chapters `Lazy functions` and `Special keys` to the `keys.rst` and `Third-party bars` to `screens.rst`.
No, I mean the adding a Gap for 3rd party bars. We implement X's struts (hopefully correctly), so that all you should have to do is fire up your 3rd party bar and qtile will move things accordingly. If this is necessary for a particular 3rd party bar, we should either fix our struts implementation or file a bug with that bar.