Many thanks @Keet for your help.
the trick with "properties" is unfortunately the same as with "open with"
The properties window looks like this:
under "ouvrir avec" i see the same list as earlier with "open with". No ability to set any path or any script.
If sweethome3d doesn't appear in this list of installed applications, no chance...
the desktop used by Elementary OS is Pantheon. Unfortunately it seems to be poorer than the desktop you use under Debian.
The executable file for sweethome3d is installed under /opt/SweetHome3D-7.1/SweetHome3D
