Preserve the current selection in "Add Columns" and "Show Columns"

The selection in "Show Columns" is approximate, and fails in the case
where a column appears more than once.  Guaranteedly selecting the
correct row would require re-deriving the new row number from the last
button pushed -- which, while possible, would require repetition of
code, and is unnecessary in most cases.

Fixes I#16972.
Warnings avoidance for when no "Show Columns" is selected
Merge branch '4.2/preserve-format-select' into 4.2-trunk