Make sure navigation events are not triggered by Fabulous

[TimLariviere]

Fixes #947

This PR does a couple things:

• Mark NavigationPage's onPopped, onPushed and onPoppedToRoot as obsolete
• Introduce NavigationPage onBackNavigated
• Allow Fabulous to catch up with XF NavigationPage in case NavPage already popped a page Fabulous was not aware of

The main issue with #947 was all NavigationPage events were both triggered by user actions and programmatic changes made by Fabulous. There was no way to differentiate who was triggering the events to be able to react only to the user's actions.

To prevent this, I created CustomNavigationPage which has an internal counter for the number of times Fabulous pops pages. This counter is used to suppress the Popped events triggered programmatically by Fabulous. If the pop has been done by the user, then BackNavigated is triggered instead.

[TimLariviere]

I tried it against the repro branch and it was working perfectly well once I replaced onPopped with onBackNavigated

[TimLariviere]

I was wondering that too, but I want to believe that Xamarin.Forms triggers events sequentially 🙃