Reproducing ReactiveUI issue 770

This creates a simple project, with two routable view models. Add
breakpoints inside the WhenActivated calls inside both HomeViewModel and
AnActivityViewModel.

Steps to reproduce:
- Breakpoint hit in HomeViewModel.WhenActivated [expected]
- Click "Go forward"
- Breakpoint hit in HomeViewModel.WhenActivated [unexpected]
- Breakpoint hit in AnActivityViewModel.WhenActivated [expected]
- Click "Go back"
- Breakpoint hit in AnActivityViewModel.WhenActivated [unexpected]
- Breakpoint hit in HomeViewModel.WhenActivated [expected]

As can be seen, there are two unexpected calls, upon deactivation, to
WhenActivated.