ScPositionHelper provides the ability to insert (and remove) row-
position pairs where the position is in pixel and related to the
spreadsheet top.

In this way one can compute a new row position by starting from the
nearest row presents in this cache-like data structure.

It offers also the ability to invalidate the cache by removing all
cached data below a given row or position.

This data structure can be used for columns, too.

Change-Id: Ifd66a003680ef9a3babf46ae952221c04451be9a
Reviewed-on: https://gerrit.libreoffice.org/40447Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Marco Cecchetti <mrcekets@gmail.com>