NavigationBar does not have a default Back icon on non-mobile platforms

[kazo0]

On Android and iOS the native bars are rendered which display the platform's native back button icon. For the platforms that use the XAML version of the NavigationBar, we should figure out if we can package an image asset in the library or perhaps use something like a SymbolIcon in the default NavigationBar style for non-mobile platforms. Although, this may not work in Windows as things get messy when trying to use IconElements within a style

Sample App
EmptySampleApp.zip