Remove concept of pausing

Removing this now will simplify the transition to TimeSources. A simplified
pause funcitonality will later be added to TimeSource. Also remove superfluous
TimedItem._timeDrift.
Implement TimeSources

Note that this patch does not use the latest naimng proposals. Animations are
not renamed to TimedEffect to avoid code churn. The new top-level object is
named AnimationController. Names will be updated in a later patch once
finalized.

- Add updateInheritedTime and deal with effectiveParentTime()
- Rename _parentToGlobalTime() to _inheritedTimeToDocumentTime()
- Add AnimationController and TimeSource
- Update tests where parent behaviour has changed (other mechanical updates to
  tests to use TimedItem.attach() are in a later commit).
Fix tests to use TimedItem.attach()
Reinstate pausing

This is now provided on AnimationController.