xdg-open is more robust in choosing default application for particular
file type: it supports fallback if the preferred application isn't
working, and most importantly it support system-wide defaults
(/usr/share/applications/defaults.list,
/usr/share/applications/mimeapps.list), so no "random" application is
chosen.
By default xdg-open tries to use environment-specific tool, like
gvfs-open - which isn't good for us, because many such tools do not wait
for editor/viewer termination. That would mean that DisposableVM would
be destroyed just after opening the file.
To avoid such effect, we set DE=generic.
FixesQubesOS/qubes-issues#1621
dca5265958