The Subsurface icon has been renamed, the .desktop must be adjusted.

This commit simply adjust the .desktop file to reflect the change in the name for the
icon (from subsurface.svg to subsurface-icon.svg).

Fixes #55

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>