Improved popup dismiss options

[reck1610]

Currently, PopupOptions.OnTappingOutsideOfPopup will only get invoked when the popup is dismissed.
If CanBeDismissedByTappingOutsideOfPopup is set to false PopupOptions.OnTappingOutsideOfPopup will not get invoked.
Furthermore, PopupOptions.OnTappingOutsideOfPopup will get invoked if the back button is pressed on Android.
I don't like this.

I propose the following changes:

• PopupOptions.OnTappingOutsideOfPopup should be renamed to PopupOptions.OnDismiss
• a new PopupOptions.OnTappingOutsideOfPopup should be created an get fired regardless of CanBeDismissedByTappingOutsideOfPopup
• a new PopupOptions.OnBackButtonPressed should be created
• a new CanBeDismissedByBackButtonPressed

CanBeDismissedByTappingOutsideOfPopup and CanBeDismissedByBackButtonPressed would only change the dismissal behavior.
PopupOptions.OnTappingOutsideOfPopup and PopupOptions.OnBackButtonPressed can be used to implement custom behavior.
PopupOptions.OnDismiss can be used to get the current behavior.

I have already implemented such a change in https://github.com/reck1610/CommunityToolkit.Maui/tree/improve-popup-dismiss

[reck1610]

My main problem with the current behavior is, that it is not possible to only prevent dismissal by tap outside of popup.