Test for dynamic table headers

This tests the dynamic table headers created by sethead="yes"
Re-enable xi:include

xi:include is quite handy if you need modular layout files.

xi:include is handled by the xml parser (luxor), but support
is needed from the layout schema as well. 

A strange message in the oxygen xml editor from jing
requires an optional xml:base attribute on the Include
element.
Documentation / schema update
XPath normalize-space: small fix and docu added

- The function now also changes newlines to spaces.
- Added short description in the XPath documentation.
New XPath function sd:dummytext()

The new function returns more than 50 words of the dummy text
"Lorem ipsum" which can be used to test the layout without
real data.