When we call org.freedesktop.Application.Open to activate
an application and pass file uris, the application may not
be able to see the files due to a flatpak sandbox.

Flatpak puts the flatpak app-id in the  X-Flatpak key in
desktop files that it exports, so we can easily recognize
applications that may be affected by this.

In this case, call the document portal to export the files
and pass the resulting uri's instead of the original ones.

https://bugzilla.gnome.org/show_bug.cgi?id=783130