Updated french translation for v0.5.1

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 "size_checked" & "size_unchecked" description in the extension, so verbs conjugation may seems out of context.

GitHub seems to add too many commits to the pull request, but the only files modified are the addition of /_locales/fr/messages.json and the deletion of /_locales/partial/fr.json (I'm a little new to GitHub, so if there's a problem add it manually). If you want a correct changelog of the translation, here is it : https://github.com/GeoffreyFrogeye/YouTubeHTML5/commit/36b60afe7f516b06d17f98080c606c851e4ff466

Anyway, v0.5 is good job !