'Metro' style for 'Frame' control (part 2)

Provides OnNavigated() helper, needs a fix for being merged in Metro
assembly.

Explanations: I have tried to move the methods inside MetroWindowHelpers
and use #if for exclude async stuff from .NET 4.0. While both Metro
assemblies build fine, demo projects never succeed. So for the time
being I have left them in the demo page unless someone can fix it.