We need this to be customized because in the latest GNOME
the desktop file name for evince changed from `evince.desktop` to
`org.gnome.Evince.desktop`.
Additonally we don't have pantheon-mail so we have to switch that
out for geary.
Also note that we're using the desktop specific type
of mimeapps.list[0]. This makes the defaults only apply for the
pantheon desktop.
[0]: https://specifications.freedesktop.org/mime-apps-spec/latest/ar01s02.html
