Nicer registration mechanism for node categories. The lists of node categories and items are now stored in a dictionary with an identifier key, so they can be registered and unregistered individually. The Add menu is now persistent and gets extended with a draw function for each of the registered node category lists.

This allows pynodes to define their own list of node categories and items and register it at runtime without interfering with the standard nodes.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56215 954f8c5b-7b00-dc11-b283-0030488c597c