send out active viewlet change event

fixes #8060

src/vs/workbench/services/viewlet/common/viewletService.ts

@@ -5,6 +5,7 @@
 'use strict';
 
 import {TPromise} from 'vs/base/common/winjs.base';
+import Event from 'vs/base/common/event';
 import {IViewlet} from 'vs/workbench/common/viewlet';
 import {createDecorator, ServiceIdentifier} from 'vs/platform/instantiation/common/instantiation';
 
@@ -13,6 +14,8 @@ export const IViewletService = createDecorator<IViewletService>('viewletService'
 export interface IViewletService {
 	_serviceBrand : ServiceIdentifier<any>;
 
+	onDidActiveViewletChange: Event<IViewlet>;
+
 	/**
 	 * Opens a viewlet with the given identifier and pass keyboard focus to it if specified.
 	 */