Add keywords in .desktop file

This adds a keywords entry and some keywords in the .desktop file.

They are (should?) be used to improve application search in DEs.

http://standards.freedesktop.org/desktop-entry-spec/latest/ar01s05.html

Signed-off-by: Salvo 'LtWorf' Tomaselli <tiposchi@tiscali.it>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

subsurface.desktop

@@ -7,4 +7,5 @@ Icon=subsurface-icon
 Exec=subsurface
 Terminal=false
 Categories=Utility;
+Keywords=dive;log;divelog;divecomputer;dives;computer