timeline: Add a new "stopped" signal

The ::stopped signal is emitted when the timeline has been completely
exhausted or when the timeline has been programmatically stopped by
using clutter_timeline_stop(); the notification at the end of the
timeline run allows to write handlers without having to check whether
the current repeat is the last one, like we are forced to do when using
the ::completed signal.

Based on the patch by: Jasper St. Pierre <jstpierre@mecheye.net>

https://bugzilla.gnome.org/show_bug.cgi?id=676854