Particles: initial support for events and actions

The following nodes work now (although things can still be improved of course):
Particle Birth Event, Praticle Time Step Event, Set Particle Attribute and Execute Condition.

Multiple Set Particle Attribute nodes can be chained using the "Execute" sockets.
They will be executed from left to right.

release/scripts/startup/nodeitems_builtins.py

@@ -498,16 +498,16 @@ simulation_node_categories = [
         not_implemented_node("SimulationNodeEmitParticles"),
     ]),
     SimulationNodeCategory("SIM_EVENTS", "Events", items=[
-        not_implemented_node("SimulationNodeParticleBirthEvent"),
-        not_implemented_node("SimulationNodeParticleTimeStepEvent"),
+        NodeItem("SimulationNodeParticleBirthEvent"),
+        NodeItem("SimulationNodeParticleTimeStepEvent"),
         not_implemented_node("SimulationNodeParticleMeshCollisionEvent"),
     ]),
     SimulationNodeCategory("SIM_FORCES", "Forces", items=[
         NodeItem("SimulationNodeForce"),
     ]),
     SimulationNodeCategory("SIM_EXECUTE", "Execute", items=[
-        not_implemented_node("SimulationNodeSetParticleAttribute"),
-        not_implemented_node("SimulationNodeExecuteCondition"),
+        NodeItem("SimulationNodeSetParticleAttribute"),
+        NodeItem("SimulationNodeExecuteCondition"),
         not_implemented_node("SimulationNodeMultiExecute"),
     ]),
     SimulationNodeCategory("SIM_NOISE", "Noise", items=[