Add API for modification of embedded supported subshaders

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Data/LightweightPipeline/LightWeightUnlitSubShader.cs

     [Serializable]
     public class LightWeightUnlitSubShader : IUnlitSubShader
     {
-        Pass m_UnlitPass = new Pass()
+        Pass m_UnlitPass = new Pass
-            PixelShaderSlots = new List<int>()
+            PixelShaderSlots = new List<int>
             {
                 UnlitMasterNode.ColorSlotId,
                 UnlitMasterNode.AlphaSlotId,

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Data/MasterNodes/PBRMasterNode.cs

 using System;
-using System.Collections.Generic;
 using System.Linq;
 using UnityEditor.Graphing;
 using UnityEditor.ShaderGraph.Drawing.Controls;
                 AlphaThresholdSlotId
             }, true);
 
-
-            if (m_SubShaders.Count == 0)
-                m_SubShaders.Add(new LightWeightPBRSubShader());
+            if (!subShaders.Any())
+                AddSubShader(new LightWeightPBRSubShader());
         }
     }
 }

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Data/MasterNodes/UnlitMasterNode.cs

 using System;
+using System.Linq;
 using UnityEditor.Graphing;
 using UnityEditor.ShaderGraph.Drawing.Controls;
 using UnityEngine;
                 AlphaThresholdSlotId
             });
 
-            if (m_SubShaders.Count == 0)
-                m_SubShaders.Add(new LightWeightUnlitSubShader());
+            if (!subShaders.Any())
+                AddSubShader(new LightWeightUnlitSubShader());
         }
     }
 }

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Data/Nodes/MasterNode.cs

     public abstract class MasterNode<T> : AbstractMaterialNode, IMasterNode where T : class, ISubShader
     {
         [NonSerialized]
-        protected List<T> m_SubShaders = new List<T>();
+        List<T> m_SubShaders = new List<T>();
 
         [SerializeField]
         List<SerializationHelper.JSONSerializedElement> m_SerializableSubShaders = new List<SerializationHelper.JSONSerializedElement>();
         public override PreviewMode previewMode
         {
             get { return PreviewMode.Preview3D; }
+        }
+        
+        public IEnumerable<T> subShaders
+        {
+            get { return m_SubShaders; }
+        }
+
+        public void AddSubShader(T subshader)
+        {
+            if (m_SubShaders.Contains(subshader))
+                return; 
+
+            m_SubShaders.Add(subshader);
+            onModified(this, ModificationScope.Node);
+        }
+
+        public void RemoveSubShader(T subshader)
+        {
+            m_SubShaders.RemoveAll(x => x == subshader);
+            onModified(this, ModificationScope.Node);
         }
 
         public string GetShader(GenerationMode mode, string outputName, out List<PropertyCollector.TextureInfo> configuredTextures)