Major overhaul of the French language pack for EE 2.5.5

Deleted 8 files (they should probably be made available in some kind of
legacy environment somewhere):
[- addons_lang /!\ EDIT: my bad, this file shouldn't have been deleted! Putting it back ASAP]
- blogger_api_lang
- bystrom_pagination_lang
- clickstats_lang
- edit_lang
- help_lang
- mt_import_lang
- publish_lang
- upload_lang

Translated 8 new files never translated before:
- expressionengine_info_lang
- fieldtypes_lang
- file_lang
- publish_tabs_custom_lang
- rte_lang
- safecracker_lang
- updated_sites_lang
- xmlrpc_lang

Added back to the French language files all untranslated items found in
the English language files.

Corrected numerous translations in all files.

All files saved as UTF8 with no BOM, LF endings.

Enjoy!