Use the newer, smarter, xclose rather than the crude gxkill

xclose just closes window instead of killing its owner, which would tear
down all this owner's windows along.

Uses different utilities with a different set of drawbacks, though:

 *  wmctrl(1) needs to be installed.

 *  [slop][1] can be optionally installed from source but can be unreliable
    when selecting some windows, e.g. urxvt.

 *  If slop is missing, xwininfo is used, which, unlike xkill(1), has no
    way of cancelling the user action (r-click selects a window just the
    same as l-click).  We work around it using timeout(1), which is
    rather inconvenient, though.

But all in all, we're still better off than with xkill(1).

  [1]: https://github.com/naelstrof/slop

dotfiles/i3/config

 bar {
         status_command i3status
         bindsym button2 exec thunar
-        bindsym button3 exec gxkill
+        bindsym button3 exec "$spn xclose"
         separator_symbol "•"
         # other suggestions:
         #     ‧•┇╻╹╹╿═╎╏◆◾║︙⎪∣∕⟘⟙∙∼⦀⦑⦒⦚⧸⧹▓▒░⧻⧺┆˘͡͡ːˌ˷｜･፧።