ui: clarify usage of GST time unit macros

g_timeout_add and clutter_actor_animate expect milliseconds but GST_SECOND is
in nano. so to fix the units differnce use glib's G_TIME_SPAN_MILLISECOND
(which is 1000) to increase code readability.

https://bugzilla.gnome.org/show_bug.cgi?id=675353