Proper detailed render statistics

Renderman has perfect detailed statistics in a form of an xml file, which you open in a browser and see it with a nice interface. So I suggest to implement something similar.

The Blender GUI mockup is quite simple for starters. After that we can add more features like overriding or incrementing the files, for example. Or maybe add this option as file output in the compositor.

This is a quite crucial piece of information for big productions.

Here is how the statistics xml looks: