herbstluftwm: improve panel startup and add autokill-hooks

Save pids of each panel in order to kill each process in case of
herbstluftwm restart/reload.

config/herbstluftwm/autostart

@@ -189,10 +189,24 @@ herbstclient set tree_style '╾│ ├└╼─┐'
 # or simply:
 # hc detect_monitors
 
+# add external panel
+{
 	# find the panel
 	panel=~/.config/herbstluftwm/panel.sh
 	[ -x "$panel" ] || panel=/etc/xdg/herbstluftwm/panel.sh
+
+	# initialize array to store panel-pids
+	pids=( )
+
+	# start panel on every monitor
 	for monitor in $(herbstclient list_monitors | cut -d: -f1) ; do
-    # start it on each monitor
+		# start it on each monitor and save pids
 		"$panel" $monitor &
+		pids+=( $! )
 	done
+
+	# wait until the panels should be stopped
+	herbstclient -w '(quit_panel|reload)'
+	# stop all started panels
+	kill ${pids[@]}
+} &