Change slice syntax such that positions appear in parentheses

This allows to ignore slice declarations using `#define slice(x)`, which
mill be useful for custom mesh material shaders, allowing to write them
such that they can be used standalone in a mesh viewer but also as slices
for OpenClonk, in which case lighting and color modulation will be applied
automatically.
Rename some shader files and add basic shader for meshes
Add possibility to add vertex slices from the parser
Make C4ShaderCall use a const C4Shader*
Pick up a shader slice only after a newline

Otherwise a workaround such as "#define slice(x)" would not work for non-OC
shader code.
Add named slice positions for vertex shaders
Make mesh shaders use the C4Shader slice machinery
Update documentation for custom shaders