Add TimedItem.clone()

This is required by the current version of the spec and will be required to
support TimedItem.attach() (part of the TimeSources implementation) in the case
that the TimedItem is already attached.
Rename TimedItem._getActiveAnimations() to _getItemsInEffect()

This method gets all TimedItems which are currently in effect. This is a
superset of the TimedItems in their active interval, as a TimedItem can have an
effect outside its active interval due to fill.

Also add a stub implementation of this method to TimedItem, since
AnimationGroup._getItemsInEffect() calls _getItemsInEffect() on its children,
which are of type TimedItem.
Rename TimedItem.updateTimeMarkers() to _updateTimeMarkers()