convert HelloAnimation to the new animation system

[stephengold]

This addresses a single piece of issue #1452.

To blend smoothly between animations, I used actionBlended(), but the resulting "halt" action is much longer than 0.5 seconds. To avoid ignoring keypresses during the "halt" action, I used simpleUpdate() to initiate the animation. If someone knows a better way to blend animations in the new system, let me know!

[tlf30]

Perhaps the blending issue is related to #1303?

[stephengold]

Not directly related, but both contributing to my impression that the new animation system was never quite completed.

Here's an idea: make Action.setLength() public. Then we could use halt.setLength(0.5) to truncate the Action to its desired duration.