ViewDestroy and ViewDisappearing not called in WPF
## 🐛 Bug Report

<!--- Summary description of the bug --->

As I posted about in StackOverflow here: [https://stackoverflow.com/questions/64595489/mvvmcross-7-how-to-allow-the-viewmodel-to-detect-the-application-is-closing?noredirect=1#comment114250689_64595489](StackOverflow), when using a WPF application, the ViewDestroy and ViewDisappearing are not being called when the application closes.


### Expected behavior

Those events should be triggered.

### Reproduction steps

Add the ViewDestroy and ViewDisappearing overrides to the TipCalc.core viewmodel.  Run the program and close the program via the upper right Window's close button.  The two events are never called.

### Configuration

**Version:** 7.x

**Platform:** 
- [ ] :iphone: iOS
- [ ] :robot: Android
- [X ] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [ ] :monkey: Xamarin.Forms
