Materials: add Gui shader

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

@@ -44,6 +44,7 @@ public class Materials {
     public static final String PBR          = "Common/MatDefs/Light/PBRLighting.j3md";
     public static final String PARTICLE     = "Common/MatDefs/Misc/Particle.j3md";
     public static final String BILLBOARD    = "Common/MatDefs/Misc/Billboard.j3md";
+    public static final String GUI          = "Common/MatDefs/Gui/Gui.j3md";
 
     /**
      * A private constructor to inhibit instantiation of this class.