This adds an embedded node tree to the simulation data block dna.
The UI in the `Simulation Editor` has been updated to show a list
of simulation data blocks, instead of individual node trees.
The new `SpaceNodeEditor.simulation` property wraps the existing
`SpaceNodeEditor.id` property. It allows scripts to get and set
the simulation data block that is being edited.
Reviewers: brecht, mont29
Differential Revision: https://developer.blender.org/D7301
