Bug 1102831 - [UserDictionary] Implement user dictionary UI to KeyboardSettingsApp

Per offline discussion, please impl four callbacks to the views:

* WillEnter
* Entered
* WillLeave
* Left

We also don't want to trigger any reflow during the transition (between willEnter and Entered, for example). That actually means either WillEnter need to be wait for a promise ("populate" in UserDictionaryView, for example) to return before start, or the view should defer any DOM changes after Enter callback.