Re-add specialization file to samples.

For some reason Xcode lost track of where it was.
Only fixes the red text; still compiled and ran fine before.
Remove finished motions at the end of Timeline::update.

Instead of the beginning.
Allows us to reason more easily about the state of the timeline.
Thread-safe timeline manipulation.

Mutex guards changes to the timeline items and item queue.
Atomic updating flag.

All things manipulated by the timeline should probably still live
on the same thread to avoid changing out from under users.

Performance (in a single-threaded context) doesn't seem to be
affected significantly.