Blender Git Loki

Made the update callback in bNodeTree registerable in RNA, this was still missing for pynodes. This update callback can be used for "global" updates of the node tree as a whole, as opposed to "local" updates of individual nodes. Any kind of update that takes node connections into account (such as dependency sorting) and does not just work on a single node should be done in the nodetree.update function rather than node.update.