fix(Column Resizing): Don't update widths in colDef

column.drawnWidth wasn't being updated after column resizing. This change
does two things:

1. Prevent updating column.width in `updateColumnDef` if it already has a
value set and the value is a number. If the column has a width that's a
number then we're assuming the user has said "I want it to be this wide"
and we shouldn't override that.
2. In the column resizer feature, update column.width, not colDef.width.