Fix a couple of tests after 1508f9
Use requestAnimationFrame() to avoid timeouts in tests
TimedItem.currentTime should be null if the inherited time is null
Remove TimedItem.itemTime and use equivalent currentTime instead

No functional change
Make TimedItem.currentTime read-only

This removes the need for a time drift in TimedItem.
Rename TimedItem.currentTime to localTime

No functional change.
Merge pull request #165 from steveblock/localTime

Fix TimedItem.localTime