Ability to export animation script from interactive GUI

[GenevieveBuckley]

napari/napari#448 (comment)

you could imagine applying the same movie to 2 different viewers... and render two animations with perhaps some data changing, etc...

This is currently pretty easy to achieve if you scripted the napari-animation controls, but much more difficult if you used the interactive GUI widget in napari. If it were possible to export the results of the interactive keyframes, this would be very useful for applying the same animation to different datasets.