Make delay of preview animation configurable

[twig]

Because the current animation preview is way too fast (30ms per frame), I've added a spinner into the animation preview panel.

It lets the user configure the delay in milliseconds (defaults to 500) for each frame/delay.

I've got no experience with jswing positioning so the position isn't in the same row as the play/pause button. If it isn't too much trouble, can you adjust it after a merge?

[darkFunction]

Looks great! Thanks. I'll move the spinner this week when I get a spare minute.

[DannyCork]

would be great to get this pr accepted. thanks