French translation update (part 1/2)

Lots of small fixes and some stylistic changes following a complete proofreading
of the existing translation. The last part of Episode 1 has not been proofread yet.

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First and biggest part of the French translation update following my proofreading. Sadly the diff is hardly readable, since Transifex broken the line endings and put all strings on one line.

Just thinking about it now, maybe we could push the current fr.po from the main repo to a dummy Transifex project/resource, then download it and push it to git (thus breaking the line endings but with no content change). I could then apply my changes on top of it and the diff would be way easier to go through.