Remove “Close Buffer”.

The “Close Buffer” item was replaced by “Close Tab”, which was already added in commit 863bba4.
Replace “Close Buffer” by “Close Tab” in all the translations.
Remove duplicate “Close Tab” translation.

The translation removed is the translation of “Close Buffer”, while the other is the translation of “Close Tab”.
Update and improve the French translation.

Changes include:

- Addition of translations for all the menu items present in default.json but absent from the previous translation.
- Correction of some mistakes.
- Typographical changes.
- Consistency or style changes.
Merge pull request #21 from rastus-vernon/patch-1

Merge #21 Replace "Close Buffer" by "Close Tab" from @rastus-vernon
Fix #21 #22 merge conflict
Update README.md
Prepare 1.15.0 release