The NodeLink template used for the shader nodes currently uses its own
method of listing nodes. To unify this with the standard node categories
system in the 'Add' menu, a couple of new operators have been added in
python.

These operators work with context arguments 'link_node'/'link_socket'
which specify a node socket that should be replaced.

There are a couple of features that are not easily generalizable:

• To determine nodes which are suitable for replacing an existing node

there are a couple of python methods (input/output_link_items,
is_link_compatible) in bpy_types. These should become RNA methods
eventually.

• Copying parameters from one node to another is not implemented yet.

The C code uses fairly specialized code for this. It would be good to
have a general mechanism for this, not just a custom python function.

• The UI for the C template is also quite specialized and probably not

reproducable down to the last detail.