labnag: add --keyboard-focus option

labnag: add --keyboard-focus option

The new `--keyboard-focus [none|on-demand|exclusive]` option (default:
`none`) allows to some keyboard controls in labnag:

  Right-arrow or Tab: move the button selection to the right
  Left-arrow or Shift-Tab: move the button selection to the left
  Enter: press the selected button
  Escape: close labnag

The selected button is highlighted with the inner 1px border. Maybe we can
instead use different colors for the selected button, but I prefer the
inner border for now because it doesn't require us to add new color
options or make them inherit labwc's theme.