Combine add_colname_to_list and add_gvalue_to_slist into single function add_value_to_vec.

The two lists were always used together so replace them with a single
vector of std::pair<std::string, std::string>; this also gets rid of the
intermediate GValue which was used to convert the returned value to a string.
operator<<() can do that for us more transparently.

Also template most of the add_value_to_vec functions.

src/backend/sql/gnc-commodity-sql.cpp

@@ -314,8 +314,7 @@ load_commodity_guid (const GncSqlBackend* be, GncSqlRow* row,
 static GncSqlColumnTypeHandler commodity_guid_handler
 = { load_commodity_guid,
     gnc_sql_add_objectref_guid_col_info_to_list,
-    gnc_sql_add_colname_to_list,
-    gnc_sql_add_gvalue_objectref_guid_to_slist
+    gnc_sql_add_objectref_guid_to_vec
   };
 /* ================================================================= */
 void