Extending the way WinRt Game Class can be created

Instead of providing just the type of the class extending Xna Game, you
can provide a factory method that you provide the customizated
implementation.
With this change, you will be able to create a Winrt Game as follows:

static void Main()
{
var factory = new

MonoGame.Framework.GameFrameworkViewSourceByDelegate<EngineStuff>(GameFactory);
///you provide a factory method
Windows.ApplicationModel.Core.CoreApplication.Run(factory);
}

///the method that provides the Game implementation
static EngineStuff GameFactory(CoreApplicationView voceview,
IActivatedEventArgs args)
{
InitialEngineDescription desc = InitialEngineDescription.Default();
desc.useMipMapWhenPossible = true;
EngineStuff engine = new EngineStuff(ref desc, LoadScreen);
return engine;
}