Reactive: Support generated bindableviewmodels

[nickrandolph]

Navigation should be able to detect when there is a generated bindableviewmodel that should be wired up as the data context for the view.

Currently the work around is to define the viewmap using the bindableviewmodel but this also requires navigating to the bindableviewmodle. Both viewmap and navigation should be based on the authored viewmodel, with the bindableviewmodel being used internally by the navigation framework