Simple model with possibility to add new rows in GTF data.

The rows in GTF_DATA structure are now stored in two ways : a table and a single linked list. The linked list allows one to add rows at any place. When a GTF_DATA structure is created, the rows are first stored in the list. The table is made at the end, when the list is complete. This update increases a bit the loading time of GTF but not too much. Another consequence is that there is now a pointer called "next" in the GTF_ROW structure that points to the next row. This pointer is useless for the clients of the library but is exported in the GTF_DATA.