String changes: control center

[Seth Nickell <snickell stanford edu>]

Currently the capplet's descriptions in the .desktop files are in
disarray. I would like to change capplet descriptions in control-center
to useful descriptions that follow the HIG's recommended verb phrasing.

Impact on translation quality should be minimal since the worst that
would happen is the old descriptions get used in some translations, and
at best more useful strings would be in many/all translations

-Seth