Supported swipe back when NavigationBar is hidden (iOS)

[grahammendick]

Fixes #664

@RichardLindhout is migrating from RNN to the Navigation router. Migrating all his custom navigation bars over to the Navigation router's native NavigationBar is too much in one go. So he wants to turn off the native navigation bar and use his own for now. But setting hidden to true on the NavigationBar prevents the swipe back gesture on iOS.

There's a difference between setNavigationBarHidden on UINavigationController and setHidden on the UINavigationBar. The first disables the swipe but the second doesn't. I'm not sure if it's official iOS behaviour (it still works on iOS 16).

If the NavigationBar is hidden checked for backTitle to decide which way to hide the navigation bar. If there's a backTitle used setHidden to keep the swipe.

// Swipe disabled
<NavigationBar hidden />

// Swipe enabled
<NavigationBar hidden backTitle="something" />

The original plan was to use the presence of the NavigationBar to determine whether to dis/able the swipe. But the Navigation router doesn’t support a StatusBar without a NavigationBar. Couldn’t hide the navigation bar, keep the swipe and customise the status bar.

While investigating this original plan found a fast way to check for no NavigationBar at all using the NSNotificationCenter. Firing a notification that the NavigationBar receives avoids having to search through all descendant views.

Even though the PR moved away from this plan, kept the NSNotificationCenter in place. Ever since Fabric stole tags, wanted a fast way to find the NavigationBar (replacing viewWithTag). For example, it’s possible to put the NavigationBar at the bottom of the page (Android CoordinatorLayout examples can put it at the bottom) which would’ve meant trawling through all views to find it.

[RichardLindhout]

It works, thank you!