[Bf-codereview] string socket type (issue 6827049)

[dfelinto at gmail.com]

Reviewers: bf-codereview_blender.org, lukas.toenne1, brechtvl,

Description:
OSL Node needs a string socket to get string parameters (e.g.
filepaths).

1) This patch adds the necessary changes in Blender BUT not in Cycles.
For Cycles I think we need to also change:
cycles_xml.cpp and graph.h, graph.cpp

But I prefer to keep the patch clean at first

2) It's not clear to me what is the point of the function:
node_add_input_from_template (on node_socket.c)

So for now I'm passing "" to the string socket. To pass the real value I
would need to add a char  property in bNodeSocketTemplate

3) to test it you can use the file (instructions in the file):
http://www.pasteall.org/blend/17320

4) if someone prefer to do the change inloco and commit the patch +
changes + cycles change, please go ahead

Please review this at https://codereview.appspot.com/6827049/

Affected files:
   intern/cycles/blender/blender_python.cpp
   source/blender/editors/space_node/drawnode.c
   source/blender/editors/space_node/node_templates.c
   source/blender/makesdna/DNA_node_types.h
   source/blender/makesrna/intern/rna_nodetree.c
   source/blender/nodes/NOD_socket.h
   source/blender/nodes/intern/node_socket.c