Fix regression caused by backward compatibility support in data source

We should only enforce the check whether a data source has set the
actions for drag on drop on a selection if it's at least version 3. For
backward compatibility we used to set a default action which would
ensure that a version <3 and >3 client can interact with each other. But
due to that the action was set to a default value which breaks any
selection. Sorry about that.

This change ensures the backward compatibility behavior does not break
selection.

As the regression causes all clients to quit it is a severe regression
which requires fast action. Due to that I'm doing a maintainer push
without review. I encourage everyone to do a post commit review.

Sorry for not noticing the regression before. The backward compatibility
was the last thing I added in that patch set and apparently I did not
run all tests again.