Created by Jerry Wang, last modified on Apr 12, 2014
在overview页面点back button后，框架会调用检查是否有pending的navigation restore待处理:
调用application view controller的get_main_entity方法:
history manager的restore_state方法:
点了back button之后的navigation target在view_state-rep_view里维护:
在这个例子里navigation target即为IC agent inbox view: