[Bf-codereview] Replacement of the node Add menu (issue 8366047)

[lukas.toenne at gmail.com]

Reviewers: bf-codereview_blender.org,

Description:
New implementation of the node Add menu. Also adds buttons in the node
toolbar with the same functionality.

node_add operator buttons in both (sub)menus and panels are generated
from a simple list of NodeItems. In addition to the basic node type
these also allow optional extra settings to be applied on new instances.

Node Items can be generated from callback functions as well. Currently
only used for group nodes, but could be used in other cases, such as
Image nodes.

Please review this at https://codereview.appspot.com/8366047/

Affected files:
   release/scripts/startup/bl_operators/node.py
   release/scripts/startup/bl_ui/__init__.py
   release/scripts/startup/bl_ui/space_node.py