rhbz#1043551: sw: avoid division-by-0 in Text Grid painting code

Possible to trigger with a document containing:
    style:layout-grid-base-height="0cm"

Change-Id: Id3bd1f29157b39e8a577be0b87b86236dbe5a50c
sw: add prefix to SwTextGridItem members

Change-Id: Ie086d56f01bc06de14c6fb1713a43aec8e6335f9
sw: remove that GETGRID abomination

Change-Id: Ib2299dae7c1aab2d9af992930c29381a1027141b