Add destroy method to UndoManager module

src/editor/model/Editor.js

@@ -775,7 +775,7 @@ export default Backbone.Model.extend({
     this.stopDefault();
     DomComponents.destroy();
     CssComposer.destroy();
-    UndoManager.clear().removeAll();
+    UndoManager.destroy();
     Panels.destroy();
     Canvas.destroy();
     Keymaps.destroy();

src/undo_manager/index.js

@@ -293,6 +293,12 @@ export default () => {
 
     getInstance() {
       return um;
+    },
+
+    destroy() {
+      this.clear().removeAll();
+      [em, um, config, beforeCache].forEach(i => (i = {}));
+      this.em = {};
     }
   };
 };