Renamed desktop-sharing media type to screen-sharing

Ignore-this: f7be083ba93456ce76d879c57f03eb5d

darcs-hash:20130118115925-58252-8aee03782afee7c95524c34086a9b83dd1505469
Renamed Desktop to Screen

Ignore-this: faba4a5c24b0c966019d89ddd38f4962

darcs-hash:20130118132408-58252-b49b2ab7f0d0010a85c0f1e4456c88c5e83b5870