Thread-safe interaction between Animation and AbstractPrimaryTimer

[mstr2]

This PR ensures that internal interactions between Animation and AbstractPrimaryTimer only occur on the FX application thread.

All changes to observable state will remain instantly visible for the calling thread. However, internally, interactions with AbstractPrimaryTimer are posted to the FX thread if necessary. This is not an unsurprising change, since the callback from the FX thread was always occuring at an unspecified time in the future.

To make this work, AbstractPrimaryTimer::pause/resume/nanos will have to be synchronized to ensure field visibility across threads. In the Animation class, interactions with AbstractPrimaryTimer will be encapsulated in the new nested class AnimationPulseReceiver, which also deduplicates redundant interactions with AbstractPrimaryTimer.

For example, repeatedly calling start() and stop() in quick succession may require just a single interaction with AbstractPrimaryTimer in the future (if we ended up in the running state), or no interaction at all (if we ended up in the stopped state).

From the point of view of the caller, the API works just as it worked before: all state changes of the Animation class are instantly visible, there is no surprising change.

With this proposal, play() can be called on a background thread, and the behavior of the Animation class remains the same. If you're reading getStatus() after calling play(), you will always see that the animation is currently running (even if internally, the animation hasn't yet been added to the primary timer).

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