Blender Git Loki

Depsgraph WIP: Basic support for OGDF export

This introduces initial preliminary support for OGDF export, including the
code to populate the graph, and all the supporting operator/rna stuff.
Much of this has been copied + adapted from the code for the GraphViz
exporting, though we are likely to need to adapt this later on.

* Currently, there is no layout engine used, so the exported graph won't look
like anything exists. But, at least all the data does get exported.

* Debugging Note: Crashes related to the ogdf string class when assigning to attributes
occur if you don't set the GraphAttributes flag corresponding to that attribute.