Adds an event handler that runs when playback is paused.
An element can be paused by a user clicking the gallery or slideshow's
pause button or by calling its pause() function
Notes:
When using onPause() 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 onPause(), 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 paused.