Code cleanup - remove most of the old/irrelevant node type defines to start afresh
Defining new registry "register" calls for typeinfo defs for all new types
Cleaning up Depsgraph struct (aka "toplevel")

A lot of the stuff there previously isn't really needed. We may still
restore it yet, but at least now we only have the stuff we absolutely need.
Modifying core internals to work with new Depsgraph toplevel structure
Porting ID-Node typeinfo and methods over to the new system
Start of code to initialise components for ID blocks when creating ID nodes

Currently, this only works (partially) for objects.
* There are still several aspects of this which haven't been defined yet
* Relationships between these are still missing