API: Rename {vertic,horizont}alSpacing to {row,column}Spacing

This makes it more consistent with Grid element, and with the
other properties the GridLayout has (rows,columns,Layout.row...)

Change-Id: I341a806df889a7888ac0466e07d73bb2ea90f5ef
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>