specification supports expand-tabs: characteristics in paragraph flow objects,
printable format backends in our DSSSL processors (Jade or OpenJade) do not
support it.
The committed code expands a literal tab character into literal spaces with
length of %default-tab-spacing%. In the position calculation, it assumes all
of characters have a fixed width and the number of characters in a node list
can be obtained by (string-length (data nl)). Note that this can be false
especially when different font sizes are used.
Tested by: wblock
