Adds an event handler that runs when playback is started or restarted.
An element can be played by a user clicking the gallery or slideshow's
play button or by calling its play() function.
Notes:
When using onPlay() on a gallery, make sure the
gallery is one where the isPlayable
capability is true.
If you change a gallery's type
to one that doesn't support onPlay(), your code may no longer function correctly.
handler(event: Event): void The name of the function or
the function expression to run when the element is played.