polybar: Show title of focused window at center

It turns out that showing the window title is very useful for actually
using the window manager, especially with no visual indicator through
window borders to determine which one is selected.

polybar/.config/polybar/config

@@ -14,7 +14,7 @@ background-alt              = ${xrdb:color8}
 
 [bar/main]
 modules-left                = bspwm
-modules-center              = mpd
+modules-center              = window
 modules-right               = cpu ram volume date time
 
 monitor                     = VGA-1
@@ -109,4 +109,9 @@ type                        = internal/alsa
 label-volume                =  %percentage%%
 format-volume-foreground    = ${color.background-alt}
 
+[module/window]
+type = internal/xwindow
+label-maxlen = 120
+label-empty = /usr/bin/bspwm
+
 ; vim:ft=dosini