Comment 6 for bug 493835

I wrote a fix using approach 1 in http://bazaar.launchpad.net/~gtg/gtg/drag-and-drop/revision/485

It's a separate insert_tag() method, so add_tag() can still be used for cases where the '@tag_name' text does not have to be inserted, such as if the user adds a tag by typing in the editor window.