Added undo actions

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Views/MaterialGraphView.cs

 
         void CollapsePreviews(EventBase evt)
         {
+            graph.owner.RegisterCompleteObjectUndo("Collapse Previews");
             foreach (AbstractMaterialNode node in graph.GetNodes<AbstractMaterialNode>())
             {
                 node.previewExpanded = false;
         void ExpandPreviews(EventBase evt)
         {
+            graph.owner.RegisterCompleteObjectUndo("Expand Previews");
             foreach (AbstractMaterialNode node in graph.GetNodes<AbstractMaterialNode>())
             {
                 node.previewExpanded = true;