Menu duplication settings -> into admin/build/menu

Issue Summary

It might be possible, with form_alter, to add the settings now implemented with a specially formatted textbox on admin/settings/dhtml_menu to the form for individual menu items in admin/build/menu.

This would be an additional text field (below the weight selector, for example), where the admin could enter the title of the pseudo-sub-item.

Not saying that the setting should be taken out of admin/settings/dhtml_menu; this should just be another way of configuring it. It makes sense to keep related settings close together, so administrating the site can be done with as few clicks as possible.