added disable property to Button

src/panels/model/Button.js

@@ -14,6 +14,7 @@ module.exports = Backbone.Model.extend({
     dragDrop: false,
     runDefaultCommand: true,
     stopDefaultCommand: false,
+    disable: false,
   },
 
   initialize(options) {

test/specs/panels/model/PanelModels.js

@@ -33,6 +33,10 @@ module.exports = {
         expect(obj.get('buttons').length).toEqual(1);
       });
 
+      it('Has a disable attribute with default value as false', () => {
+        expect(obj.get('disable')).toEqual(false);
+      });
+
     });
 
     describe('Buttons', () => {