Automatic numbering of XML text headers

*Title: Automatic numbering of XML text headers

*Original author: Bard Farstad

*Documentation:
A template variable {$header_number} is available in the 'header.tpl' file. It
can be used to automatically generate a link or a prefix to a specific header
item.
Very useful if you want to generate a TOC and have it link to the correct
place in the text.

It does not change the template but makes the template variable available.
For the first header it will have the value 1, the second will have 2. A sub
header under the second header will have 2_1 and so on.