Add support for auto-positioning of items in a grid layout

Items will be laid sequentially in the grid, either in LeftToRight
mode or TopToBottom mode (depending on the value of the flow property)

Change-Id: Ib080517260ac4519171e77abd533cad3f306f70e
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>