* When KonqUndoManager gets a undo() request:
    * If last operation to undo is not an open tab, then fine.
    * If last operation to undo is a tab
        * If there's no active KonqMainWindow, it will get undoed
        * Else If it's a closed tab from the active windows, it will get undoed
        * Else If it's a closed tab from an unactive window, it will test the next undo operation (recursively)