Allows for different header render strategies.
An abstract implementation of a header render strategy which is only missing the code to traverse the child hierarchy, since the sequence of that traversal is what will make the difference between the different header render strategies.
This a header render strategy implements a child->parent->root sequence, which is inverse to how it was until Wicket 1.5.
This has been Wicket's default header render strategy before WICKET 1.5 which uses
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