Add subtitles to visibility context menu items

Closes #155

Tusker/Extensions/Visibility+Helpers.swift

@@ -24,6 +24,19 @@ extension Status.Visibility {
         }
     }
     
+    var subtitle: String {
+        switch self {
+        case .public:
+            return "Everyone"
+        case .unlisted:
+            return "Hidden from public timelines"
+        case .private:
+            return "Followers only"
+        case .direct:
+            return "Mentioned users only"
+        }
+    }
+    
     var imageName: String {
         switch self {
         case .public:

Tusker/Screens/Compose/ComposeHostingController.swift

@@ -250,7 +250,7 @@ class ComposeHostingController: UIHostingController<ComposeContainerView> {
             item.accessibilityLabel = String(format: NSLocalizedString("Visibility: %@", comment: "compose visiblity accessibility label"), draft.visibility.displayName)
             let elements = Status.Visibility.allCases.map { (visibility) -> UIMenuElement in
                 let state = visibility == newVisibility ? UIMenuElement.State.on : .off
-                return UIAction(title: visibility.displayName, image: UIImage(systemName: visibility.unfilledImageName), identifier: nil, discoverabilityTitle: nil, attributes: [], state: state) { [unowned self] (_) in
+                return UIAction(title: visibility.displayName, subtitle: visibility.subtitle, image: UIImage(systemName: visibility.unfilledImageName), state: state) { [unowned self] (_) in
                     self.draft.visibility = visibility
                 }
             }
@@ -270,9 +270,9 @@ class ComposeHostingController: UIHostingController<ComposeContainerView> {
                 item.image = UIImage(systemName: "link")
                 item.accessibilityLabel = "Federated"
             }
+            let instanceSubtitle = "Only \(mastodonController.accountInfo!.instanceURL.host!)"
             item.menu = UIMenu(children: [
-                // todo: iOS 15, action subtitles
+                UIAction(title: "Local-only", subtitle: instanceSubtitle, image: UIImage(named: "link.broken"), state: localOnly ? .on : .off) { [unowned self] (_) in
-                UIAction(title: "Local-only", image: UIImage(named: "link.broken"), state: localOnly ? .on : .off) { [unowned self] (_) in
                     self.draft.localOnly = true
                 },
                 UIAction(title: "Federated", image: UIImage(systemName: "link"), state: localOnly ? .off : .on) { [unowned self] (_) in
@@ -474,3 +474,13 @@ extension ComposeHostingController: ComposeDrawingViewControllerDelegate {
         dismiss(animated: true)
     }
 }
+
+fileprivate extension UIAction {
+    convenience init(title: String, subtitle: String?, image: UIImage?, state: UIAction.State, handler: @escaping UIActionHandler) {
+        if #available(iOS 15.0, *) {
+            self.init(title: title, subtitle: subtitle, image: image, identifier: nil, discoverabilityTitle: nil, attributes: [], state: state, handler: handler)
+        } else {
+            self.init(title: title, image: image, identifier: nil, discoverabilityTitle: nil, attributes: [], state: state, handler: handler)
+        }
+    }
+}