cmVisualStudio10TargetGenerator: extend DOM-like generation

The local Elem class is extended with more XML-generating functions.
WriteGroups() is rewritten to use these new functions,
avoiding BuildFileStream.

Source/cmVisualStudio10TargetGenerator.h

@@ -158,7 +158,8 @@ private:
   void WriteEvent(const char* name,
                   std::vector<cmCustomCommand> const& commands,
                   std::string const& configName);
-  void WriteGroupSources(std::string const& name, ToolSources const& sources,
+  void WriteGroupSources(Elem& e0, std::string const& name,
+                         ToolSources const& sources,
                          std::vector<cmSourceGroup>&);
   void AddMissingSourceGroups(std::set<cmSourceGroup*>& groupsUsed,
                               const std::vector<cmSourceGroup>& allGroups);