TreeTable: column resize performance

[ray007]

When using TreeTable with a few features (scrolling, resizable columns, ...), each row is using flex layout.
Consequently, we need a style set on each cell for column width.
Resizing a column requires updating the style of a cell in each row, which is quite a performance drag if you have a lot of rows.

If instead each cell had a class col-$index or col-$fieldName, we could add a style tag where we could a style definition for the column, and the browser would do this work for us. Use css variables, and you don't even need to update the style-tag when a column width changes...

Thoughts?