Add BeforeNavigate Event handler to allow user to stop transition

[joefeser]

An example would be validation.

If validation fails, you do not want the transition to be performed.

It would be nice if the items that are passed to the event include the current page and the page the user is attempting to navigate to.

If this is the navigate to backlog item then please ignore.

[finnsson]

I added the "from page" as a fourth argument in the guard-callback. Should probably be the first argument to the guard-callback but I don't want to break backwards compatibility all the time.