Now the computation of the cell cursor position exploits the cached
row/col positions in HeightHelper/WidthHelper.

That makes navigating through arrow keys independent from the current
cell cursor position: the cell cursor position is updated at the
bottom of the document as fast as at the top even for very big
spreadsheets.

Change-Id: I2f23d4439f2157e702f09b56106bebdd0e0bbe86
Reviewed-on: https://gerrit.libreoffice.org/40450Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Marco Cecchetti <mrcekets@gmail.com>