This commit moves code in all node editor files to the
`blender::ed::space_node` namespace, except for C API
functions defined in `ED_node.h`, which can only be moved
once all areas calling them are moved to C++.
The change is fairly straightforward, I just moved a couple
of "ED_" code blocks around to make the namespace more
contiguous, and there's the method for adding a pointer to
a struct in a C++ namespace in DNA.
Differential Revision: https://developer.blender.org/D13871
