Tabbed mode for columns

[YaLTeR]

Add a toggle that switches a column from a vertical stack of windows to a vertical set of tabs. See #876 for design concepts. I think I prefer tab indicators on the left to tab indicators at the top, but that's an aside.

The cool detail of this idea is that it completely preserves keyboard navigation with no new necessary binds. You still switch up and down between windows in a column, still consume/expel, they are just rendered as tabs instead of as a stack.