Add bindings for hide/show current cussor column in columns sheet.

[blais]

Minor change adding a way to toggle to show a single column in the Columns metasheet. I'm finding myself a bit annoyed having to (gv) show all and then rehide some of the columns.

(Feel free to change the binding overrides to be more in line with the holistic choices across the tool. Thank you for this wonderful tool! :-) )