sxhkd, polybar, dunst: Add setting no disturb mode

By invoking key combination fn+F9 (XF86Search button), we can quiet
dunst notifications.

This change will be picked up by polybar and a small do not disturb
symbol is shown in the top bar (a tiny moon).

Both scripts are very simple and make use of a temporary file, so a
little brittle but workable.

sxhkd/.config/sxhkd/sxhkdrc

@@ -25,6 +25,9 @@ XF86AudioMute
 XF86Audio{LowerVolume,RaiseVolume}
   control-volume {down, up}
 
+XF86Search
+  dunstctl set-paused toggle && echo "$(dunstctl is-paused)" > /tmp/dunstpaused
+
 # Open terminal emulator (the variable gets set in sh module basic env vars)
 super + Return
   $TERMINAL