Add semantic properties for the Toolbar BackButton

[PureWeen]

Description

Currently it's tricky to modify the text you want read out for the BackButton, if you want it to differ from what you've set for the BackButtonBehavior (Shell) or the NavigationPage.SetBackButtonTitle.

We should add some additional properties to modify the semantic text for the back button.

Steps to Reproduce

Scenario 1 – Shell Navigation (GoToAsync + BackButtonBehavior)

• Launch the app — lands on MainPage Sample1.zip
• Tap "GoToAsync → DetailPage (Shell + BackButtonBehavior)"
• Navigates to DetailPage via Shell routing
• Back button visually shows "Override" (set using BackButtonBehavior.TextOverride)
• Focus the back button using VoiceOver/TalkBack

Scenario 2 – NavigationPage (PushAsync + SetBackButtonTitle)

• From MainPage, tap "PushAsync → SecondPage (NavigationPage.SetBackButtonTitle)"
• A modal NavigationPage opens, displaying NavHomePage
• Tap "PushAsync → SecondPage"
• Navigates to SecondPage — back button visually shows "BackTitle"
• Focus the back button using VoiceOver/TalkBack

Workaround

One workaround on Android would be to do this

#if ANDROID
     ToolbarHandler.Mapper.AppendToMapping(nameof(IToolbar.BackButtonVisible), (handler, view) =>
     {
          handler.PlatformView.NavigationContentDescription = "text";
     });

     ToolbarHandler.Mapper.AppendToMapping(nameof(Toolbar.BackButtonTitle), (handler, view) =>
     {
          handler.PlatformView.NavigationContentDescription = "text";
     });
#endif
})

We've added this issue to our backlog, and we will work to address it as time and resources allow. If you have any additional information or questions about this issue, please leave a comment. For additional info about issue management, please read our Triage Process.