Add back button support to NavBar

[KaneCheshire]

[KaneCheshire]

Should also add support for long-pressing on the back button which is new in iOS 14

[stefanrenne]

In our project I created this Element

struct BackBarButton: Element, Tappable {
    let id: String? = nil
    let index: Int = 0
    let parent: Element = NavBar()
    let type: XCUIElement.ElementType = .button
}