Be less cavalier about changing existing manufacturer IDs

[RikerW]

Since the editor determines how to display items by manufacturer, changing the ID of a new manufacturer silently vanishes anything that it had previously. This is the correct behavior re not displaying them, but changing the name should probably prompt a warning if you had previous items - since otherwise, they will be hidden from view but CC will still try to import them (possibly causing really cryptic errors, if the hidden items are malformed).

I'll try to work on this at some point, but this is being recorded as an issue anyway so I can remember.