DebuggerItem: Compare against original item from DebuggerItemManager

Compare against the original item from the DebuggerItemManager, not
to the item stored in the model. This will keep the change flag, even
when switching back and forth between items in the model.

Task-number: QTCREATORBUG-10954
Change-Id: I54535c45e3c3e45fabbf83e0a35c3bd674158892
Reviewed-by: hjk <hjk121@nokiamail.com>