hidden as a boolean independent of column width

From @midichef in #3148:

Currently a column is said to be hidden when it has self.width <= 0. I'd prefer to see hidden implemented as a boolean that's independent of width.

Right now, if I hide a column with an assigned width, it loses its previously assigned width. When I unhide it with gv, it gets a new different width. If hiding preserved self.width, this wouldn't happen.

@saulpw agreed: "I still kinda like the negative-width concept but it's a hack and I guess it's time we did it 'right' :)" History of the 2018 negative-width scheme: https://github.com/saulpw/visidata/issues/3148#issuecomment-4765027775

[this issue written by Claude Fable 5 and approved by @saulpw]