This also introduces the `blender::nodes` namespace. Eventually,
we want to move most/all of the node implementation files into
this namespace.
The reason for this file-move is that the code fits much better
into the `nodes` directory than in the `blenkernel` directory.
This adds new callbacks to `bNodeSocketType` and `bNodeType`.
Those are used to generate a multi-function network from a node
tree. Later, this network is evaluated on e.g. particle data.
Reviewers: brecht
Differential Revision: https://developer.blender.org/D8169
