When routing the SplitNav responder chain through the root VC, go as deep into it as possible

Makes keyboard shortcuts from, e.g., TimelineVC accessible when the root is TimelinesPageVC

See #302

Tusker/Screens/Utilities/SegmentedPageViewController.swift

@@ -198,3 +198,9 @@ extension SegmentedPageViewController: StatusBarTappableViewController {
         return .continue
     }
 }
+
+extension SegmentedPageViewController: NestedResponderProvider {
+    var innerResponder: UIResponder? {
+        currentViewController
+    }
+}

Tusker/Screens/Utilities/SplitNavigationController.swift

@@ -283,7 +283,11 @@ private class SplitSecondaryNavigationController: EnhancedNavigationViewControll
         // ordinarily, the next responder in the chain would be the SplitNavigationController's view
         // but that would bypass the VC in the root nav, so we reroute the repsonder chain to include it
         // first seems to be nil when using the view debugger for some reason, so in that case, defer to super
-        owner.viewControllers.first?.view ?? super.next
+        if let root = owner.viewControllers.first {
+            return root.innermostResponder() ?? super.next
+        } else {
+            return super.next
+        }
     }
     
     private func configureSecondarySplitCloseButton(for viewController: UIViewController) {
@@ -300,3 +304,17 @@ private class SplitSecondaryNavigationController: EnhancedNavigationViewControll
     }
     
 }
+
+protocol NestedResponderProvider {
+    var innerResponder: UIResponder? { get }
+}
+
+extension UIResponder {
+    func innermostResponder() -> UIResponder? {
+        if let nestedProvider = self as? NestedResponderProvider {
+            return nestedProvider.innerResponder?.innermostResponder() ?? self
+        } else {
+            return self
+        }
+    }
+}