VS: Add support for XAML source files

XAML files are by default of type Page in the vcxproj and can be
overriden by setting the VS_XAML_TYPE property.  The .cpp and .h file
of the same name are automatically added as depending on the XAML file.

New VSXaml test builds a basic XAML WindowsStore 8.1 app with VS2013.

Tests/VSXaml/App.xaml.h

@@ -0,0 +1,27 @@
+//
+// App.xaml.h
+// Declaration of the App class.
+//
+
+#pragma once
+
+#include "App.g.h"
+
+namespace VSXaml
+{
+    /// <summary>
+    /// Provides application-specific behavior to supplement the default Application class.
+    /// </summary>
+    ref class App sealed
+    {
+    protected:
+        virtual void OnLaunched(Windows::ApplicationModel::Activation::LaunchActivatedEventArgs^ e) override;
+
+    internal:
+        App();
+
+    private:
+        void OnSuspending(Platform::Object^ sender, Windows::ApplicationModel::SuspendingEventArgs^ e);
+        void OnNavigationFailed(Platform::Object ^sender, Windows::UI::Xaml::Navigation::NavigationFailedEventArgs ^e);
+    };
+}