check-xdg-portal: Look for more xdg-desktop-portal interfaces
The June 2023 update to Steam makes more use of the various portals, in particular the FileChooser for "native" File -> Open... dialogs. If xdg-desktop-portal does not have a backend that offers this interface, or has one but has been (mis)configured not to use it, then we should diagnose that.
@andresr, are you the person to ask about Steam's use of these portals? Are there any more that we should be checking?
Edited by Simon McVittie