🐛  Small accessibility fix to MultiGrid so that focus outline shows through by default for main (bottom/right) Grid. Top and left Grids are also not tab-focusable by default now since they are scroll-observers anyway.

✨  Added columnWidth parameter to ColumnSizer and deprecated getColumnWidth callback. The callback was not necessary since columnWidth doesn't change without a re-render and fixed number values perform better in Grid due to some internal optimizations anyway.
