Fix public currentPageIndex property name

There is a `currentIndex` public readonly property in `MWPhotoBrowser`.
There is no getter method, nor `self.currentIndex` or `_currentIndex` is assigned.
So it's always `0`.

On other hand there is a `_currentPageIndex` instance variable.
It's usefull to be able to get readonly access to this variable.

So I suggest to rename `currentIndex` property to `currentPageIndex` so it will become usefull.