Preliminary "Ribbon" Support in Avogadro

I term it as “ribbon” because it really isn’t a ribbon. I haven’t figured out how to calculate the plane the ribbon lies on and right now it is a series of cylinders drawn from point to point. I hope you will agree that it already looks fairly effective and can certainly show the nature of the secondary structure of biological molecules.

There is still a lot to do such as draw the tube in a more efficient manner as a continuous OpenGL construct and draw each residue separately. It would also be good to be able to draw ribbons but this may take a deeper understanding of ribbons and how they are visualised than I have. If there are any biologists out there who want to help me out then tips, or even better patches, would be great fully received