Trebuchet : Improve deletion of all folder contents

Rather than invoking callbacks for each removed item, just add a
callback for when all items are removed and invoke it once.

This solves the case where if a folder is updated frequently and its
visibility is toggled by the user, we don't enter an inconsistent state.

Change-Id: I149cb2218501e4d0ef0d312550e58e76bf08a850