This just drops i3-nagbar for swaynag, changes i3-msg to swaymsg, and
mentions of i3 to sway. The former even worked fine, just did weird
things with windowing.
This signposts that we've actually moved permanently (fingers crossed)
to Sway. I should have done this the moment the first
non-backward-compatible change went in so I could revert to that point,
but meh.
