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.

- The AmtkFactory class has been added, which replace `gtef_action_info_store_create_menu_item()` and removes the `GtefActionInfoStore:application` property.

- `gtef_action_info_central_store_get_instance()` has been renamed to amtk_action_info_central_store_get_singleton().