xdg-open: handle opening files when DE=flatpak
This PR allows using xdg-open
via portals on file and directory names in addition to URLs
I do wonder if the value DE=flatpak
should be renamed to something like DE=portal
or DE=container
to better reflect its usefulness outside of flatpaks (https://github.com/freedesktop/xdg-utils/pull/12 relevant), but that feels out of scope for this MR