Provide explicit handing of application/x-desktop in mimeapps.list

[emkll]

In freedomofpress/securedrop-workstation#494 (comment) , application/x-desktop files are opened in a dvm , without an explicit rule. In default VMs, they are opened in a text editor, likely because debian on qubes doesn't use all of gnome's tooling and gdm. However, to ensure defense-in-depth, we should explicitly add the association in the mimeapps.list

[conorsch]

To aid in review, recall that file and xdg-mime may report different results:

$ file --mime-type /usr/share/applications/debian-xterm.desktop
/usr/share/applications/debian-xterm.desktop: text/plain
$ xdg-mime query filetype /usr/share/applications/debian-xterm.desktop
application/x-desktop

It's the latter we're trying to ensure is handled sufficient by the Client and associated tooling.