ENH: Simplify string attributes in Xcode generator

This change cleans up the implementation of cmXCodeObject to avoid
un-escaping and re-escaping string values.  There is no need to store
the string in escaped form.  It can be escaped once when it is printed
out to the generated project file.

Source/cmXCodeObject.h

@@ -147,6 +147,8 @@ public:
   std::vector<cmXCodeObject*> const& GetObjectList() { return this->List;}
   void SetComment(const char* c) { this->Comment = c;}
 protected:
+  void PrintString(std::ostream& os) const;
+
   cmTarget* Target;
   Type TypeValue;
   cmStdString Id;