I added a handler for this SLurl type that displays the classifieds
details in the side tray.

In order to do this, you have to open the profile of the classified's
creator first. Which means that I have to ask the server for the
classified's description in order to get the creator id. This is done
with an LLAvatarPropertiesObserver observer. One complication here is
that this observer takes the avatar id as a parameter, but we don't
know that (that's what we're trying to find out) - so I made it
possible to pass in a null avatar id - it's only used to filter out
non-interested observers in the notifyObserver() call.