Updated french translation for v0.5.1

Placed directly in _locales.
I made some alterations from the original, e.g. "vidéo" (video) is replaced by "lecteur" (player) because it sounds way less weird.
I've also not seen "toggle_disabled" and "size_checked" & "size_unchecked" description in the extension, so verbs conjugation may seems out of context.