allow deletion of specific types of objects

Following on from http://drupal.org/node/142762 it should be easy to "rebuild" the entire set of aliases of a certain kind so that you can then bulk update them.

To me the best way to do this is to make it easy to delete all aliases of a certain kind of object (objects being users, nodes, terms). That enables "bulk update of an entire kind of object that replaces the old aliases" but it also enables people to just delete all of the aliases of that kind in case they want to do that separately from bulk updating. So, I think providing it separately from the bulk update option provides the greatest/widest benefit. I'd certainly be willing to hear the ideas of other folks on this issue.

Fortunately the code/query behind this is not hard, so we just need solid ideas.