Simulations: Embed simulation node tree in simulation data block

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

release/scripts/startup/bl_ui/space_node.py

@@ -151,6 +151,14 @@ class NODE_HT_header(Header):
             if snode_id:
                 layout.prop(snode_id, "use_nodes")
 
+        elif snode.tree_type == 'SimulationNodeTree':
+            row = layout.row(align=True)
+            row.prop(snode, "simulation", text="")
+            row.operator("simulation.new", text="", icon='ADD')
+            simulation = snode.simulation
+            if simulation:
+                row.prop(snode.simulation, "use_fake_user", text="")
+
         else:
             # Custom node tree is edited as independent ID block
             NODE_MT_editor_menus.draw_collapsible(context, layout)