Now the computation of the row/column headers data exploits the cached
row/col positions in HeightHelper/WidthHelper.

That makes updating row/column headers at the bottom of the document
as fast as at the top even for very big spreadsheets.

Change-Id: I27381b520aaed68662e4d07c5879046d182c70e6
Reviewed-on: https://gerrit.libreoffice.org/40451Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Marco Cecchetti <mrcekets@gmail.com>