A sliver that displays a two dimensional scrollable (in both directions) grid of box children.
The table consists of a horizontal top header, a vertical left header, and a rowsCount x colsCount
grid of cells.
The vertical header is pinned at the top edge of the table and displays colsCount cells that are
The horizontal header is pinned at the left side of the table and displays rowsCount cells that are
The grid of cells can scroll both vertically and horizontally under the top and left headers respectively.