Add a feature flag for spaces. (#4429)

* Remove the huge conditional compilation block on AppSettings.

It is still required for the ElementCallBaseURL however.

* Add a feature flag for spaces.

ElementX/Sources/Application/Settings/AppAppearance.swift

@@ -0,0 +1,26 @@
+//
+// Copyright 2023, 2024 New Vector Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+// Please see LICENSE files in the repository root for full details.
+//
+
+import SwiftUI
+
+/// Used to specify the user's app specific appearance preference
+enum AppAppearance: CaseIterable, Codable {
+    case system
+    case dark
+    case light
+        
+    var interfaceStyle: UIUserInterfaceStyle {
+        switch self {
+        case .light:
+            return .light
+        case .dark:
+            return .dark
+        case .system:
+            return .unspecified
+        }
+    }
+}