Gtef 2 -> Amtk 3: Tepl 3 Reference Manual

## Gtef 2 -> Amtk 3

- The classes and functions related to actions and menus have been moved to a new shared library called Amtk (Actions, Menus and Toolbars Kit), since it was not really related to text editors, it is more general. But Amtk is still developed inside the Tepl repository.

- Add AmtkFactory and AmtkFactoryMenu, which replace `gtef_action_info_store_create_menu_item()` and removes the `GtefActionInfoStore:application` property.