Slic3r, French i18n + improvements

Hi @zaggo,

Here are a bunch of modifications / improvements that I hope can be useful for the community.

As I said in the related issue, I've changed the Sparkle appcasts' location so you might not want to merge that. Some things may not be clean enough yet too, so do not hesitate to point out what needs refactoring, I'll gladly look at it when I can.

Credits to @miwucs for a great part of the Slic3r work.

Best, 
Cyril