Tepl 4 -> Tepl 5: Tepl 6 Reference Manual

## Tepl 4 -> Tepl 5

- The TeplMetadataManager API has been re-worked.

- The `TeplFileMetadata` class has been replaced by TeplMetadata, the latter no longer depends on TeplFile.

- The `tepl_file_get_file_metadata()` function has been removed.

- The tepl_tab_load_file() function no longer loads the file metadata (but it was anyway an implementation detail).

- The `TeplAbstractFactoryVala` class has been removed, because it's possible to manually manage GObject references in Vala.