Use UIViewController's designated initializer instead of -[init]

There is no guarantee that `-init` will always be called when creating a new UIViewController. Hence setting the default value for `_showsNavigationToolbar` should be done in the designated initializer `-initWithNibName:bundle:`.

Also, when instantiating the view controller from a storyboard, nether `init` nor `initWithNibName:bundle` are called, we must implement `initWithCoder:`.