Update taglist when tag.volatile value is changed (#3765)

The `volatile` property is supported by the taglist (theme variables
`beautiful.taglist_bg_volatile` and `beautiful.taglist_fg_volatile`)
but taglist is not updated when the property is changed.

lib/awful/widget/taglist.lua

@@ -711,6 +711,7 @@ function taglist.new(args, filter, buttons, style, update_function, base_widget)
         tag.attached_connect_signal(nil, "property::screen", ut)
         tag.attached_connect_signal(nil, "property::index", ut)
         tag.attached_connect_signal(nil, "property::urgent", ut)
+        tag.attached_connect_signal(nil, "property::volatile", ut)
         capi.client.connect_signal("property::screen", function(c, old_screen)
             u(c.screen)
             u(old_screen)