TimedItem.parentGroup should be an AnimationGroup, not a TimedItem

TimedItem attempts to use the following methods and properties of its
parentGroup member.
- iterationTime
- paused
- _childrenStateModified()
- type
- remove()
- indexOf()
- _addChild()

Of these, only the first two are provided by TimedItem. The others are added
only by AnimationGroup.
Make use of TimedItem._effectiveParentTime
Make use of ParGroup for DEFAULT_GROUP