[iOS] Allow UpdateTabBarHidden method in ShellItemRenderer to be overridden

Description

I implemented a custom action button in the Shell TabBar with a CustomShellItemRenderer derived from ShellItemRenderer (following @VladislavAntonyuk's great blog post) but an added control (i.e., action button) does not react to setting Shell.TabBarIsVisible on a page.

Making the UpdateTabBarHidden method in the iOS implementation of ShellItemRenderer a protected virtual method would allow it to be overridden by derived classes which could handle added custom controls.

Public API Changes

Make UpdateTabBarHidden in the iOS implementation of ShellItemRenderer a protected virtual method instead of (implicitly) private. This would align with the Android implementation (see: ShellItemRenderer.cs)

https://github.com/dotnet/maui/blob/c3221bf08d291c9f69070aa11d6d60f61a569aae/src/Controls/src/Core/Compatibility/Handlers/Shell/iOS/ShellItemRenderer.cs#L434 ->

protected virtual void UpdateTabBarHidden()

Intended Use-Case

This would allow for implementing a method that reacts to setting Shell.TabBarIsVisible and updates the visibility of added custom controls.

The Android implementation of ShellItemRenderer already has a protected virtual UpdateTabBarVisibility method that can be overridden in a CustomShellItemRenderer to keep the visibility of the custom action button in sync:

internal class CustomShellItemRenderer : ShellItemRenderer
{
  private Android.Widget.Button? centerView;
  ...
  // Create custom action button, i.e., centerView
  ...
  protected override void UpdateTabBarVisibility()
  {
      base.UpdateTabBarVisibility();

      if (DisplayedPage is null || centerView is null)
          return;

      centerView.Visibility = ShellItemController.ShowTabs ? ViewStates.Visible : ViewStates.Gone;
  }
}

The API change would allow something similar for iOS:

internal class CustomShellItemRenderer : ShellItemRenderer
{
  private UIButton? centerView;
  ...
  // Create custom action button, i.e., centerView
  ...
  protected override void UpdateTabBarHidden()
  {
      base.UpdateTabBarHidden();

      if (ShellItemController == null || centerView == null)
          return;

      centerView.Hidden = !ShellItemController.ShowTabs;
  }
}

dotnet / maui