Sets the ratio with which the column expands.
By default, all columns expand equally (treated as if all of them had
an expand ratio of 1). Once at least one column gets a defined expand
ratio, the implicit expand ratio is removed, and only the defined
expand ratios are taken into account.
If a column has a defined width (
overrides this method's effects.
Example: A grid with three columns, with expand ratios 0, 1
and 2, respectively. The column with a ratio of 0 is exactly
as wide as its contents requires. The column with a ratio of
1 is as wide as it needs, plus a third of any excess
space, because we have 3 parts total, and this column
reserves only one of those. The column with a ratio of 2, is as wide
as it needs to be, plus two thirds of the excess