change SetLayoutRow widths to be more CSS's fr way instead of negative numbers

[hajimehoshi]

Currently, a negative width indicates the width in the way like the right edge is positioned away from the container's right edge by the specified length. I think this is a little complicated, and I prefer fr way very much.

https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_grid_layout/Basic_concepts_of_grid_layout#the_fr_unit