Blender Git Loki

Added "ID Users" depsgraph builder, for setting up links between ID datablocks

This is useful for one-to-many situations when building relationships between
operations, but is likely to come in handy for other parts of Blender later
on.

For example, when constructing drivers on armature bones, we don't have any easy
way of referring back to the object (and thus the pose bones) to make those
depend on these as well.

Just a few skeleton types have been implemented now.