Merge of the PyNodes branch (aka "custom nodes") into trunk.

PyNodes opens up the node system in Blender to scripters and adds a number of UI-level improvements.

=== Dynamic node type registration ===
Node types can now be added at runtime, using the RNA registration mechanism from python. This enables addons such as render engines to create a complete user interface with nodes.

Examples of how such nodes can be defined can be found in my personal wiki docs atm [1] and as a script template in release/scripts/templates_py/custom_nodes.py [2].

=== Node group improvements ===
Each node editor now has a tree history of edited node groups, which allows opening and editing nested node groups. The node editor also supports pinning now, so that different spaces can be used to edit different node groups simultaneously. For more ramblings and rationale see (really old) blog post on code.blender.org [3].

The interface of node groups has been overhauled. Sockets of a node group are no longer displayed in columns on either side, but instead special input/output nodes are used to mirror group sockets inside a node tree. This solves the problem of long node lines in groups and allows more adaptable node layout. Internal sockets can be exposed from a group by either connecting to the extension sockets in input/output nodes (shown as empty circle) or by adding sockets from the node property bar in the "Interface" panel. Further details such as the socket name can also be changed there.

[1] http://wiki.blender.org/index.php/User:Phonybone/Python_Nodes
[2] http://projects.blender.org/scm/viewvc.php/trunk/blender/release/scripts/templates_py/custom_nodes.py?view=markup&root=bf-blender
[3] http://code.blender.org/index.php/2012/01/improving-node-group-interface-editing/

doc/python_api/examples/bpy.types.NodeTree.py

@@ -0,0 +1,23 @@
+"""
+Poll Function
++++++++++++++++
+The :class:`NodeTree.poll` function determines if a node tree is visible
+in the given context (similar to how :class:`Panel.poll`
+and :class:`Menu.poll` define visibility). If it returns False,
+the node tree type will not be selectable in the node editor.
+
+A typical condition for shader nodes would be to check the active render engine
+of the scene and only show nodes of the renderer they are designed for.
+"""
+import bpy
+
+
+class CyclesNodeTree(bpy.types.NodeTree):
+    """ This operator is only visible when Cycles is the selected render engine"""
+    bl_label = "Cycles Node Tree"
+
+    @classmethod
+    def poll(cls, context):
+        return context.scene.render.engine == 'CYCLES'
+
+bpy.utils.register_class(CyclesNodeTree)