Moves to the previous image or slide.
The previous() function returns a Promise that is resolved when the previous
image or slide is completely rendered. Calling previous() when on the first item
moves to the last item.
Notes:
When using previous() 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 previous(), your code may no longer function correctly.