Symlinked existing icons to complete MATE integration

Hi,

I'm one of the MATE Desktop team. This pull request adds compatibility for the MATE Desktop to Numix Circle.

I've been using the Numix Circle icons for some weeks and noticed that some icons were missing a Numix Circle variant. Fortunately, all the required icons exist in Numix Circle and it was just of matter of creating the appropriate sym-links.

Regards, Martin.