Layer to support methods expected by Renderers

Hey guys - I am going to try something. I am worried that I have been too pig headed about this. The real motivation for Layer was to make the constructors make sense - not to make Andrea's life difficult. I think with my object-oriented hat on, wanting each class to have a minimal responsibility, I have made things too difficult / risky.

I am going to look at pulling the getStyle() and getFeatureSource() and getQuer() methods up into Layer (and having them return "empty" content). I can explain in the javadocs that these methods are used by feature based renderers; and the default implementation only needs to be overridden if your Layer has something to say.

I would hope this would avoid the amazing amount of casts witnessed in Andrea's and Micheal's patches. I don't think it impacts on the clarity of the interfaces; the constructors are still clear; we still have concrete implementations each of which has their own specific "workarounds" for providing a FeatureSource - and since it is not tired up in conditional code I hope it would be readable.