Invoke INavigationAware callbacks on UI thread

[elliot-gawthrop]

Pull request type

Please check the type of change your PR introduces:

• Update
• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes

What is the current behavior?

INavigationAware callbacks are invoked on the thread pool, and exceptions are silently discarded.

Issue Number: #1306

What is the new behavior?

• Callbacks are invoked on the UI thread.
• Unhandled exceptions will break when thrown if debugging in Visual Studio, then bubbled up to the AppDomain.CurrentDomain.UnhandledException event.

Other Information

I have also removed the now-unnecessary UI dispatching code from the demo and gallery base view models.