Remove DEFAULT_GROUP.currentState()

This is used only for logging and will be superfluous after the switch to
TimeSources.
Fix intrinsic duration

- Provide an intrinsic duration for TimedItems and fix the value
- Make it a 'package private' method
- Add a test
Fix override behavior of TimedItem.duration property

Explicitly setting TimedItem.duration should override
TimedItem.Timing.duration, until explicitly set to undefined.
Fix updating TimedItem.animationDuration

This should always reflect TimedItem.duration.
Fix override behavior of TimedItem.animationDuration

Setting this property should override the calculated value, until it is
explicitly set to undefined.
Fix updating TimedItem.endTime

TimedItem.endTime should be updated in response to changes to startTime,
duration and animationDuration. This change fixes this and adds a test.
Make TimedItem.endTime read-only
Fix re-layout of parallel parent when updating child endTime

This patch adds a number of tests for this re-layout, and fixes the case where
the child's animationDuration is updated.
Add tests for updating children in a sequence group
Remove superfluous update of parent when setting currentTime.

The parent is not affected by the child's local time, so this update is not
required.
Merge pull request #64 from steveblock/modifyChildrenTest

Clan-up, bug fixes and new tests that need doing before I can refactor things to use TimeSources