handle w:gridBefore by faking cells (fdo#38414)

Docx's w:gridBefore means that there should be this given space in the table
grid before any cells come. But writer requires tables to be rectangular, so
the space needs to be faked using cells without border. So far so good, but
now reality in the form of the retarded overdesigned writerfilter comes.
The internal representation of table data (and not just one actually) is
pretty non-obvious and hard to modify, seems to be modelled just to follow
the parser data the way it comes. Moreover dmapper gets notified of w:gridBefore
only after cells in the row have been already processed. So after futile attempts
to add the fake cells somehow in dmapper I've eventually given up and hacked up
input handling to fake input as if the fake cells were actually there (which
was tedious to find out as well, but at least it's reasonably doable).

Change-Id: I7107e13f28dd3f7093688782f64238167cead76f