It's still a bit simple-minded. Most software will keep the first bound
fixed and move the second. Lines currently has the bounds in a queue of
sorts. But I have a test to indicate the behavior that is definitely
desired. We'll see if we need it to get more complex.
I spent some time trying to reduce the duplication between identical
operations with and without the shift key pressed. However it makes
things harder to understand. If you try to process selection in
App.keychord_pressed in main.lua, you have to sometimes process the
selection before (e.g. including the state of the cursor _before_ an
arrow key takes effect), and sometimes after (e.g. copying the selection
before resetting it, even though it doesn't include a shift key)
Let's just leave things as they are.
