guide grid for table column width

[lutz_bendlin]

After disabling auto size for column widths, I still need to have some sort of guide to align my custom column width. Very often I show multiple tables vertically, and I would like to align the widths of the corresponding columns. If I had a vertical guide line when changing the width of a column I could use that to align the column with the same column in the table above it, for example.