Springboard-like page scrolling

Added Springboard-like page scrolling for when the collection view has paging enabled and updated the example. We needed this for collection views that can only scroll to the page offset, and also it feels more "natural".

This feature required an architectural change: `currentView` is now a sibling of the collection view instead of its child. This avoids overcomplicated calculations to move `currentView` with the page change animation. Also, this should work better when the collection view is smaller than its parent.

All the page calculations were added to a separate `UICollectionViewFlowLayout` category, as they are independent of `LXReorderableCollectionViewFlowLayout`'s logic.

Totally understand if you don't want this change in master.