Fix folder-menupopup binding in mailnews/base/content/folderWidgets.xml to not duplicate its items after Customize dialog is closed. This happens e.g. in the "Get new mail" menu or "Go -> Folder". This is currently worked around in mailCore.js::MailToolboxCustomizeDone() but it is ugly and not future proof (other items with the binding may appear in the future and not notice to be added into this workaround). This attempts to fix problems from bug 520457, bug 534448, bug 709733 in a better way.