Node Group Interface

This patch implements one of the main features described in my recent code.blender.org blogpost [1]

It replaces the current lists of sockets on either side of the group editing window by internal "proxy nodes". This is avoids long node connections from the sides of a node group and provides a place to edit all the details of a node group interface socket. It also simplifies code by removing the need to take additional socket lists into account. In the future the proxy nodes could be used for creating external interfaces for regular (non-group) node trees as well.

User wiki documentation can be found here [2]. (I noticed there is currently no wiki docs section on general node editing features, beside specifics for shader/compositor nodes. Would be a good idea to make one and integrate this info there.)

NOTE: This version of the patch does not include changes to startup.blend for size reasons, which means the proxy node theme color will be black by default.

[1]
http://code.blender.org/index.php/2012/01/improving-node-group-interface-editing/

[2]
http://wiki.blender.org/index.php/User:Phonybone/NodeTreeInterface

[lukas.toenne, 2012-02-09 16:26:24]

Removed proxy node ntree storage, this causes problems with do_versions +
lib_link