Save column reference in the column width property

This is to assign an issue number for future references to source code history.

In the wake of PR #136 (closed) for issue #401 (closed): adding or removing columns to/from the History view should not interfere with the association between a column and its width.

A solution for making the property independent of the position of a column could be to add an indication of the column to each width value saved to the property.

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