Support geometry and TCS shaders

Add fullvalues option: ply filenames are listed explicitly, and do not
use the <timestep>-<iso> nomenclature.
Add cutplanes option with two parameters: nearcut, farcut.
Add CuttingPlane value to menubutton. If enabled, the button will
provide a cutting plane which moves smoothly between neacut and farcut
and back.
Support http(s) based config files.
Use two-faced normals in iso shader.
Send 'A' for selected atom in multiuser mode.
Add optional chain affiliations to atoms (chainfile option).