Refine services description in Info.plist

This splits the service offerings in two: One for files, one for text selections. The use of NSSendFileTypes limits the service to only showing up in Finder for supported filetypes. The text selection service description is changed to match the description TextEdit uses for its "Open selection" service. Note that the file service is only compatible with 10.6+.
Merge the two service entries into one.

Turns out you can't have the same keyboard shortcut for two items, even if they'll only ever be active in different contexts. Switching back to a single entry solves that. All the other improvements still work, we just don't have two separate labels.
Merge pull request #119 from Westacular/services-fix

Refine services description in Info.plist