Running desktop file which contains % in the exec key doesn't works correctly
$ dex -d /usr/share/applications/kde4/kwalletmanager.desktop
Executing command: kwalletmanager %u
As explain here: http://standards.freedesktop.org/desktop-entry-spec/latest/ar01s06.html
A command line may contain at most one %f, %u, %F or %U field code. If the application should not open any file the %f, %u, %F and %U field codes must be removed from the command line and ignored.
So dex should remove %u in the above example.
implement field codes. closes issue #8