removed Mortar scope from navigation drawer scene

We were using the Scope to take advantage of its onExitScope() method and show the next scene when the navigation drawer was detached from the window. This turned out to be unnecessary -- instead, there is a public method, NavigationDrawerScene.Presenter.showNextScene(), that is called after Presenter.dropView().

Also cleaned up some minor stuff like packages, and created a MortarScene in case we want to use it in the future.