[rc/{sway,i3status}] display screen icon on the bar when nosleep

- since i3status doesn't support custom blocks, we have to wrap around
  it to add more info to it.  if colours aren't needed, it could be as
  simple as something like this (from the i3status manual):

          #!/bin/sh
          # shell script to prepend i3status with more stuff

          i3status | while :
          do
                  read line
                  echo "mystuff | $line" || exit 1
          done

  but since i like colours, we need a more sophisticated solution
  which involves dealing with json; and thus the output_format needs
  to be "i3bar".

- `killall -USR1 i3status` forces an update when we toggle nosleep.