urlopen: Add a more clever xdg-open executable
When inside a Steam Runtime container we currently make xdg-open
points to the flatpak-xdg-utils implementation that relies on
xdg-desktop-portal. But if an user doesn't have a working
xdg-desktop-portal it will fail.
Additionally the steam:// URLs might end up opening another instance
of the Steam client, even if one instance was already running. This can
happen if more than one version are installed, e.g. the Steam client
from the distro repositories and the Flatpak version.
With this alternative implementation we try a more clever approach that
should workaround the xdg-open shortcomings that we experienced in our
Steam related use cases.
This includes !280 (merged), probably better to review and merge the other one first.
Edited by Ludovico de Nittis