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