Implement App#command_loop,refresh_loop with <q> to quit.

Extract `App#start` refresh loop into `App#refresh_loop`, implementing
`App#command_loop` for key presses. Use threads. Get next key press, if
it exists, with `App#next_key` hook, in turn calling
`Display::Screen#next_key`. Respond to key presses in `App#do_command`.

Wire up first command `<q>`, which quits the application gracefully (as
does `<Ctrl>+<c>`), demonstrating command usage as well as providing a
more intuitive shutdown.