MediaGallery models a list of GalleryDate-based sections (in practice, months), each of which has a certain number of items. Sections are loaded on demand (that is, there may be newer and older sections that are not in the model), and always know their number of items. Items are also loaded on demand, potentially non-contiguously. This model is designed around the needs of UICollectionView (a thread's All Media view), but it also supports flat views of media (swiping between items in the media detail view, which can cross album boundaries).
This model is a result of refactoring I did last year to improve the performance and UX of the All Media view, but the implementation in MediaGallery.swift is entangled with the actual attachments and messages in the database, making it hard to test. This commit pulls the state management part out into its own struct, MediaGallerySections, which wraps the OrderedDictionary of items-by-section. Our MediaGalleryItem model references TSAttachmentStream directly and our queries go straight to the database, so to avoid having to set up real attachments in testing, MediaGallerySections is generic over a loader (delegate? data source?) of items with "gallery dates" and unique IDs. Now all the basic section- and item-loading APIs can be unit-tested.
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