VS: Add support for WinRT project properties (#12930)

VS_WINRT_EXTENSIONS: Boolean property that correspond to "Enable
Tailored Features" in Visual Studio 11 IDE.

VS_WINRT_REFERENCES: Semicolon-delimited list of *.winmd references to
add to the project, which creates a new <ItemGroup>.

Source/cmVisualStudio10TargetGenerator.h

@@ -48,6 +48,7 @@ private:
   void WriteProjectConfigurationValues();
   void WriteCLSources();
   void WriteDotNetReferences();
+  void WriteWinRTReferences();
   void WriteObjSources();
   void WritePathAndIncrementalLinkOptions();
   void WriteItemDefinitionGroups();