Add a variable in hlwm theme for font height

Use it to fix bar height after font size change.

xdg-config-dir/herbstluftwm/autostart

@@ -163,8 +163,7 @@ hc unlock
 hc detect_monitors
 
 if [ -f ~/.config/herbstluftwm/herbstpanel.sh ]; then
-	font_height=13
-	bar_height=$(($font_height))
+	bar_height=$(($hlwm_font_height))
 	for m in $(hc list_monitors | cut -d : -f 1); do
 		hc pad "$m" 0 0 $bar_height 0
 		hc monitor_rect "$m" | {

xdg-config-dir/herbstluftwm/theme

@@ -30,3 +30,5 @@
 
 : ${hlwm_font="-misc-fixed-medium-r-normal--14-130-75-75-c-70-iso10646-1"}
 : ${hlwm_face="Fixed:pixelsize=14"}
+
+: ${hlwm_font_height=14}