use vsnprintf instead of snprintf in string_printf function

Also, revert to old location: Utils.hpp and utils.cpp

src/libslic3r/Utils.hpp

@@ -93,6 +93,8 @@ namespace PerlUtils {
     extern std::string path_to_parent_path(const char *src);
 };
 
+std::string string_printf(const char *format, ...);
+
 // Standard "generated by Slic3r version xxx timestamp xxx" header string, 
 // to be placed at the top of Slic3r generated files.
 std::string header_slic3r_generated();