Add stacked section controller demo

Summary:
Adding a pretty basic stacked demo. 3 different sections, each with 3 child section controllers that handle an `Int` data.

Requires #354 to be fully functional (working range).

Closes #134

- [x] All tests pass. Demo project builds and runs.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes https://github.com/Instagram/IGListKit/pull/355

Differential Revision: D4363859

Pulled By: jessesquires

fbshipit-source-id: 5dbc8e18388fed4930f189e3477ff57f0edf2d13

Examples/Examples-iOS/IGListKitExamples/ViewControllers/DemosViewController.swift

@@ -36,6 +36,7 @@ final class DemosViewController: UIViewController, IGListAdapterDataSource {
         DemoItem(name: "Supplementary Views", controllerClass: SupplementaryViewController.self),
         DemoItem(name: "Self-sizing cells", controllerClass: SelfSizingCellsViewController.self),
         DemoItem(name: "Display delegate", controllerClass: DisplayViewController.self),
+        DemoItem(name: "Stacked Section Controllers", controllerClass: StackedViewController.self),
     ]
 
     override func viewDidLoad() {

Examples/Examples-iOS/IGListKitExamples/ViewControllers/StackedViewController.swift

@@ -0,0 +1,59 @@
+/**
+ Copyright (c) 2016-present, Facebook, Inc. All rights reserved.
+
+ The examples provided by Facebook are for non-commercial testing and evaluation
+ purposes only. Facebook reserves all rights not expressly granted.
+
+ THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ FACEBOOK BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+import UIKit
+import IGListKit
+
+final class StackedViewController: UIViewController, IGListAdapterDataSource {
+
+    lazy var adapter: IGListAdapter = {
+        return IGListAdapter(updater: IGListAdapterUpdater(), viewController: self, workingRangeSize: 1)
+    }()
+    let collectionView = IGListCollectionView(frame: .zero, collectionViewLayout: UICollectionViewFlowLayout())
+
+    let data = [128, 256, 64]
+
+    override func viewDidLoad() {
+        super.viewDidLoad()
+        collectionView.backgroundColor = UIColor(white: 0.95, alpha: 1)
+        view.addSubview(collectionView)
+        adapter.collectionView = collectionView
+        adapter.dataSource = self
+    }
+
+    override func viewDidLayoutSubviews() {
+        super.viewDidLayoutSubviews()
+        collectionView.frame = view.bounds
+    }
+
+    // MARK: IGListAdapterDataSource
+
+    func objects(for listAdapter: IGListAdapter) -> [IGListDiffable] {
+        return data as [IGListDiffable]
+    }
+
+    func listAdapter(_ listAdapter: IGListAdapter, sectionControllerFor object: Any) -> IGListSectionController {
+        // note that each child section controller is designed to handle an Int (or no data)
+        let sectionController = IGListStackedSectionController(sectionControllers: [
+            WorkingRangeSectionController(),
+            DisplaySectionController(),
+            HorizontalSectionController(),
+            ])!
+        sectionController.inset = UIEdgeInsets(top: 0, left: 0, bottom: 20, right: 0)
+        return sectionController
+    }
+
+    func emptyView(for listAdapter: IGListAdapter) -> UIView? { return nil }
+
+}