VS: Refactor handling of "header" sources in VS >= 10

Create a WriteHeaderSource method to handle writing of sources
classified by cmGeneratorTarget as header files.  This will be
useful to add special VS-specific handling for some headers.

Source/cmVisualStudio10TargetGenerator.h

@@ -57,6 +57,7 @@ private:
   void WriteProjectConfigurations();
   void WriteProjectConfigurationValues();
   void WriteMSToolConfigurationValues(std::string const& config);
+  void WriteHeaderSource(cmSourceFile const* sf);
   void WriteExtraSource(cmSourceFile const* sf);
   void WriteSource(std::string const& tool, cmSourceFile const* sf,
                    const char* end = 0);