with this snapshot, the app will crash immediately after run, however, I want to commit it to repository from preventing code lost.
what is done: created a new custom table view controller and the controller conforms to a delegate protocol which is designed for the add-to-do item view controller to communicate with the table view controller.
Revert "with this snapshot, the app will crash immediately after run, however, I want to commit it to repository from preventing code lost."

This reverts commit 7d987e26ddf165efd0a5b66d71d0e637f301283b.
新增了一个navigation controller和一个被那个navigation controller管理着的table view controller -- 作为to do item lists。它可以通过model segue来实现跳转场景
用自定义的类型来代表那个table view controller，然后给add todo item view controller的add和cancel buttons添加了unwind segue，这需要先在destination view controller中新增一个unwind action，unwind segue在运行时判断谁是destination view controller时需要这个action，并且当unwind segue被触发时，这个action会被segue调用。
新增了：能够让to do item list table view 通过在 unwind action中获取 add item view中的textfield中（一个outlet，并且对这个textfield应用了auto layout）的内容，然后显示在table view中。