This implements a new builtin node tree type called `SimulationNodeTree`.
It is not yet embedded in the `Simulation` data block.
The node tree will initially be used for the new particle nodes system.
When the cmake option `WITH_NEW_SIMULATION_TYPE` is enabled, a new
`Simulation Editor` is shown in the editors menu (which is just a node editor).
This patch does not add entries to the Add Node menu, so it is empty.
Reviewers: brecht
Differential Revision: https://developer.blender.org/D7287
