Update Materials: add Particle shader

jme3-core/src/main/java/com/jme3/material/Materials.java

@@ -38,10 +38,11 @@ package com.jme3.material;
  */
 public class Materials {
 
-    public static final String UNSHADED = "Common/MatDefs/Misc/Unshaded.j3md";
-    public static final String LIGHTING = "Common/MatDefs/Light/Lighting.j3md";
-    public static final String PBR = "Common/MatDefs/Light/PBRLighting.j3md";
     public static final String SHOW_NORMALS = "Common/MatDefs/Misc/ShowNormals.j3md";
+    public static final String UNSHADED     = "Common/MatDefs/Misc/Unshaded.j3md";
+    public static final String LIGHTING     = "Common/MatDefs/Light/Lighting.j3md";
+    public static final String PBR          = "Common/MatDefs/Light/PBRLighting.j3md";
+    public static final String PARTICLE     = "Common/MatDefs/Misc/Particle.j3md";
 
     /**
      * A private constructor to inhibit instantiation of this class.