It is working fine except that it crashes at the end when running
procedure:new_return_values(). I have not understood why yet, but it may
be a bug in lua LGI. Maybe we'll have to create the returned
GimpValueArray by hand in Lua.
Also the dialog does not show the source yet.

This is not installed yet (and lua interpreter is not checked at
configure) because of these issues which I will have to figure out. In
the meantime, I commit this work-in-progress.