Update french translation

Hi!

I discovered Ultracopier today and I was really happy to see that it works with Windows 7.
I missed Supercopier (I was using it few years ago when I was on Windows XP) and I was searching for an active project to replace it.
I hope Ultracopier will do the job!

So here is a small contribution: I updated the French translation to remove spelling mistakes.
I didn't mark all items as Done because I'm not satisfied with some of them (it's easy to spot them with Qt Linguist).

Please tell me if something is wrong, I'll try to fix it :)