Add destroy method to Panels module

src/editor/model/Editor.js

@@ -771,7 +771,7 @@ export default Backbone.Model.extend({
     DomComponents.destroy();
     CssComposer.destroy();
     UndoManager.clear().removeAll();
-    Panels.getPanels().reset();
+    Panels.destroy();
     Canvas.destroy();
     Keymaps.destroy();
     RichTextEditor.destroy();

src/panels/index.js

@@ -28,7 +28,6 @@
 import defaults from './config/config';
 import Panel from './model/Panel';
 import Panels from './model/Panels';
-import PanelView from './view/PanelView';
 import PanelsView from './view/PanelsView';
 
 export default () => {
@@ -233,6 +232,13 @@ export default () => {
       });
     },
 
+    destroy() {
+      panels.reset();
+      panels.stopListening();
+      PanelsViewObj.remove();
+      [c, panels, PanelsViewObj].forEach(i => (i = {}));
+    },
+
     Panel
   };
 };