Update ToggleControl to UIElements

Cosmetic fixes to
- InvertColors node
- ChannelMixer node
- Blend node

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Data/Nodes/Artistic/Adjustment/InvertColorsNode.cs

         [SerializeField]
         private bool m_RedChannel;
 
-        [ToggleControl("Red")]
+        [ToggleControl("R")]
         public Toggle redChannel
         {
             get { return new Toggle(m_RedChannel, channelCount > 0); }
         [SerializeField]
         private bool m_GreenChannel;
 
-        [ToggleControl("Green")]
+        [ToggleControl("G")]
         public Toggle greenChannel
         {
             get { return new Toggle(m_GreenChannel, channelCount > 1); }
         [SerializeField]
         private bool m_BlueChannel;
 
-        [ToggleControl("Blue")]
+        [ToggleControl("B")]
         public Toggle blueChannel
         {
             get { return new Toggle(m_BlueChannel, channelCount > 2); }
 
         private bool m_AlphaChannel;
 
-        [ToggleControl("Alpha")]
+        [ToggleControl("A")]
         public Toggle alphaChannel
         {
             get { return new Toggle(m_AlphaChannel, channelCount > 3); }

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Data/Nodes/Artistic/Blend/BlendNode.cs

         [SerializeField]
         BlendMode m_BlendMode = BlendMode.Overlay;
 
-        [EnumControl("")]
+        [EnumControl("Mode")]
         public BlendMode blendMode
         {
             get { return m_BlendMode; }

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Controls/ChannelMixerControl.cs

         float m_Maximum;
         bool m_Initialized;
 
-        int m_UndoGroup = -1;
-
         public ChannelMixerControlView(string label, float minimum, float maximum, AbstractMaterialNode node, PropertyInfo propertyInfo)
         {
             m_Node = node;

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Controls/ToggleControl.cs

 using System;
 using System.Reflection;
+using UnityEditor.Graphing;
+using UnityEditor.Experimental.UIElements;
-using UnityEditor.Graphing;
+using UnityEngine.Experimental.UIElements.StyleSheets;
+using UnityEditor.ShaderGraph;
 
 namespace UnityEditor.ShaderGraph.Drawing.Controls
 {
         GUIContent m_Label;
         AbstractMaterialNode m_Node;
         PropertyInfo m_PropertyInfo;
-        IMGUIContainer m_Container;
+
+        UnityEngine.Experimental.UIElements.Toggle m_Toggle;
 
         public ToggleControlView(string label, AbstractMaterialNode node, PropertyInfo propertyInfo)
         {
                 throw new ArgumentException("Property must be a Toggle.", "propertyInfo");
-            m_Label = new GUIContent(label ?? ObjectNames.NicifyVariableName(propertyInfo.Name));
-            m_Container = new IMGUIContainer(OnGUIHandler);
-            Add(m_Container);
+            
+            label = label ?? ObjectNames.NicifyVariableName(propertyInfo.Name);
+
+            var value = (Toggle)m_PropertyInfo.GetValue(m_Node, null);
+            var panel = new VisualElement { name = "togglePanel" };
+            if (!string.IsNullOrEmpty(label))
+                panel.Add(new Label(label));
+            Action changedToggle = () => { OnChangeToggle(); };
+            m_Toggle = new UnityEngine.Experimental.UIElements.Toggle(changedToggle);
+            m_Toggle.SetEnabled(value.isEnabled);
+            panel.Add(m_Toggle);
+            Add(panel);
+            var value = (Toggle)m_PropertyInfo.GetValue(m_Node, null);
+            m_Toggle.SetEnabled(value.isEnabled);
+ 
-                m_Container.Dirty(ChangeType.Repaint);
+            {
+                Dirty(ChangeType.Repaint);
+            } 
-        void OnGUIHandler()
+        void OnChangeToggle()
+            m_Node.owner.owner.RegisterCompleteObjectUndo("Toggle Change");
-
-            using (var changeCheckScope = new EditorGUI.ChangeCheckScope())
-            {
-                m_Container.SetEnabled(value.isEnabled);
-
-                bool isOn = EditorGUILayout.Toggle(m_Label, value.isOn);
-                value = new Toggle(isOn, value.isEnabled);
-                if (changeCheckScope.changed)
-                {
-                    m_Node.owner.owner.RegisterCompleteObjectUndo("Change " + m_Node.name);
-                    m_PropertyInfo.SetValue(m_Node, value, null);
-                }
-            }
+            value.isOn = !value.isOn;
+            m_PropertyInfo.SetValue(m_Node, value, null);
+            Dirty(ChangeType.Repaint);
         }
     }
 }

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Resources/Styles/MaterialGraph.uss

 ChannelMixerControlView > #buttonPanel {
     flex-direction: row;
     flex-grow: 1;
+    margin-bottom: 6;
-    margin-left: 2;
-    margin-right: 2;
+    margin-left: 0;
+    margin-right: 0;
     align-items: center;
 }
 
     margin-right: 2;
     min-width: 30;
     max-width: 30;
+}
+
+ToggleControlView > #togglePanel {
+    flex-direction: row;
+    flex-grow: 1;
+}
+
+ToggleControlView > #togglePanel > Label {
+    flex-grow: 1;
+    padding-left: 0;
+    padding-right: 0;
+    padding-top: 0;
+    padding-bottom: 0;
+    margin-left: 8;
+    margin-right: 8;
+    margin-top: 4;
+    margin-bottom: 4;
+}
+
+ToggleControlView > #togglePanel > Toggle {
+    margin-left: 8;
+    margin-right: 8;
+    margin-top: 4;
+    margin-bottom: 4;
+    align-self: center;
 }
 
 ObjectControlView {