chunk-common.xsl: header.navigation

The template "header.navigation" in chunk-common.xsl creates the navigation
header of each chunked page. It consists of the "next" and "prev" links on the
left and right side of the page.

Additionally in the center links for "up" and "home" are generated which some
people for whom I do the XSL customization want to be interchanged ("row1" and
"row2"). Of course I can override this template to get the desired result but I
wonder if it would be a good idea to make this configurable because quite a few
people have already asked about it.

Michael
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