Merge eric/wallpaper-and-settings-improvements xfdesktop branch?

[Jérôme Guelfucci]

Hello all,

Eric has been working on polishing the xfdesktop-settings UI in this 
branch and reached a point where it should be OK to merge his work.

This branch also improves the "different wallpaper for each workspace" 
feature he introduced in the git master branch and allows to turn it off.

Could you please check it out and give your opinion on this?

Cheers,

Jérôme