Toolbar implementation. Tried with a base content 
page but didn't work. Maybe a good thing as toolbar can be different per page.
The toolbar has its own view model and it gets initialized through an extension method

NavigationService now supports NavigateBack but doesn't allow receiving specific parameters
to pass to an eventual subscriber. Will implement later.

Need to check a better way to filter messages for subscribers. At this point, it doesn't seem to filter
on a topic but only on the type of message.