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.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55981 954f8c5b-7b00-dc11-b283-0030488c597c