true if the toolbar should be hidden, false if it should be visible.
When a UIKit.UIViewController is a child to a UIKit.UINavigationController, it may display a toolbar at the bottom of the screen. If that is not the desired behavior, the application developer should set this property to true.
If a UIKit.UIViewController sets this to true, the toolbar remains hidden until the UIKit.UIViewController is popped from the stack. This will effect other UIKit.UIViewControllers that are currently visible.