Sets the widths/heights, which is a list of numbers separated
by comma to denote the width/height of each cell in a box.
It is the same as
#setHeights.
For example, "10%,20%,30%" means the second cell shall
occupy 10% width, the second cell 20%, the third cell 30%,
and the following cells don't specify any width.
Note: the splitters are ignored, i.e., they are not cells.
Another example, ",,30%" means the third cell shall occupy
30% width, and the rest of cells don't specify any width.
Of course, the real widths depend on the interpretation of
the browser.