Currently there are many function declarations in BKE_node.h that
don't actually have implementations in blenkernel. This commit moves
the declarations to NOD_composite.h, NOD_texture.h, and
NOD_shader.h instead. This helps to clarify the purpose of the
different modules.
Conceptually, the node type defines should probably be moved to DNA,
but it's questionable whether that change is worth it at this point.