Blender Git Loki

Embed simulation node tree in simulation data block

This also shows the node tree in the Simulation Editor.
There is a new operator to add a simulation.

One debatable aspect of this patch is how I integrated the `SpaceNodeEditor.simulation`
property in RNA. I decided to wrap the existing `id` property, because the description
says `Data-block whose nodes are being edited`, which is exactly what is happening here.

Differential Revision: https://developer.blender.org/D7301