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/nodeitems_builtins.py

@@ -58,6 +58,13 @@ class TextureNodeCategory(SortedNodeCategory):
                 context.space_data.tree_type == 'TextureNodeTree')
 
 
+class SimulationNodeCategory(SortedNodeCategory):
+    @classmethod
+    def poll(cls, context):
+        return (context.space_data.type == 'NODE_EDITOR' and
+                context.space_data.tree_type == 'SimulationNodeTree')
+
+
 # menu entry for node group tools
 def group_tools_draw(self, layout, context):
     layout.operator("node.group_make")
@@ -70,6 +77,7 @@ node_tree_group_type = {
     'CompositorNodeTree': 'CompositorNodeGroup',
     'ShaderNodeTree': 'ShaderNodeGroup',
     'TextureNodeTree': 'TextureNodeGroup',
+    'SimulationNodeTree': 'SimulationNodeGroup',
 }
 
 
@@ -467,17 +475,28 @@ texture_node_categories = [
     ]),
 ]
 
+simulation_node_categories = [
+    # Simulation Nodes
+    SimulationNodeCategory("SIM_GROUP", "Group", items=node_group_items),
+    SimulationNodeCategory("SIM_LAYOUT", "Layout", items=[
+        NodeItem("NodeFrame"),
+        NodeItem("NodeReroute"),
+    ]),
+]
+
 
 def register():
     nodeitems_utils.register_node_categories('SHADER', shader_node_categories)
     nodeitems_utils.register_node_categories('COMPOSITING', compositor_node_categories)
     nodeitems_utils.register_node_categories('TEXTURE', texture_node_categories)
+    nodeitems_utils.register_node_categories('SIMULATION', simulation_node_categories)
 
 
 def unregister():
     nodeitems_utils.unregister_node_categories('SHADER')
     nodeitems_utils.unregister_node_categories('COMPOSITING')
     nodeitems_utils.unregister_node_categories('TEXTURE')
+    nodeitems_utils.unregister_node_categories('SIMULATION')
 
 
 if __name__ == "__main__":