linux-fat: Fix up fat linux install icons, mime types and .desktop files

linux-fat: Fix up fat linux install icons, mime types and .desktop files

use the standard xdg utilities to install mime type and .desktop
files. The standard xdg icon installer doesn't handle svg icons, so we
have to do those manually.

With this patch, xfce, gnome3 and kde4 all place the applications in
user-visible menus, display correct file icons and launch correct
applications for our file types.

Signed-off-by: Keith Packard <keithp@keithp.com>