A BoardGrid object is a collection of RowGrid objects. Each RowGrid object's horizontal property is true. Adjacent RowGrid objects are positioned one on top of the other.
Properties:
containerId: a unique integer identifying a container of Card objects.
defStack: the default CardStack object
count: the no. of RowGrid objects
rows: a list of RowGrid objects of length equal to the count property
dimensions: width, height (in pixels)
position: left, top (in pixels)
offset: distance in pixels between adjacent RowGrid objects (height = offset multiplied by count)
visible: if false, BoardGrid is not visible to user