Gets the index of the gallery's current item.
The indices of the items in a gallery are zero-based. For example, if
the third item is currently being displayed, then currentIndex
returns 2.
Notes:
The currentIndex property is supported by galleries where the
hasCurrentItem capability is true.
If you change the gallery's type
to one that doesn't support currentIndex and you try to use it, your code may no longer function correctly.