Add support for items order by zIndex

`UICollectionViewLayout` supports the ability to order its elements (cells, supplementary views & decoration views) not only by their IndexPath, so items with small IndexPath will be in the collectionView subviews' stack in front of item with bigger IndexPath. This is required, for example, if you want to have an animation on a cell's content which will hover above all other cells.

It'd be great if MagazineLayout will support changing item's zIndex.

I can implement it & make a pull request. I think that the best place to get this data is part of the delegate call for 'MagazineLayoutItemSizeMode' (maybe we need to change its name to MagazineLayoutItemSizeAndOrderMode)