Press a keybinding to adjust the width/height of a pair of panes.

This comes with extra work - we have to have each parent pane track the relative size of each child, and then when the window is resized, recursively resize each control to match the absolute size to the previous proportions. Currently, we're just using two row/cols of equal width.

Start with some simple amount, like 5% at a time. Adjusting by a single char at a time would only be possible if both panes were the same font size, which isn't guaranteed.