The timeline frame function emits several signals, assuming that the
timeline will exist after these signals return. This assumption can be
invalid if signal handlers unref the timeline.
https://bugzilla.gnome.org/show_bug.cgi?id=668675
Otherwise, we end up doing maximal work with minimal effect, and
bring the X server to 100% CPU. Note that this makes spinner widgets
look really unclear, but they really have to be changed to adapt
to enable-animations == FALSE at the widget level.
This will only be used internally by GtkStyleContext in order
to animate state changes. When a proper animation framework is
introduced in GTK+, it may be ditched.
