Merge pull request #266 from dstockwell/update-state

Don't add the reference element to an absent parent
Merge pull request #275 from dstockwell/remove-step-timing

Stop exporting StepTimingFunction and SplineTimingFunction
Merge pull request #276 from dstockwell/placeholders

Add placeholders for missing interfaces
Test that styles and timing are visible after a player seek
Repeat the last tick at last tick time when active animation state changes.

This includes changes to:

 * Players which have ticked at least once.
 * The animation tree (including changing the source of a Player)
 * Specified timings

A repeat is not triggered by changes to AnimationEffects.
Rename Animation.animationEffect to effect and use a bool in exitModifyCurrentAnimationState
Make depth counting exception safe
Test chaning player start time
Use timed_test API
More explanation and checks for animation state tracking