Fixes for multi-window support. (#5528)

* Correctly handle the re-opening of the main window.

Add an additional safe-guard to ensure only one main window exists.
Make sure all secondary windows use the correct tint colour.

* Fix a bug where the settings screen isn't shown on macOS when the AppLock feature is enabled.

ElementX/Sources/Screens/Settings/SettingsScreen/SettingsScreenModels.swift

@@ -6,8 +6,7 @@
 // Please see LICENSE files in the repository root for full details.
 //
 
-import Foundation
-import UIKit
+import SwiftUI
 
 enum SettingsScreenViewModelAction: Equatable {
     case close
@@ -51,6 +50,8 @@ struct SettingsScreenViewState: BindableState {
     
     let isBugReportServiceEnabled: Bool
     
+    let navigationBarVisibility: Visibility
+    
     var bindings = SettingsScreenViewStateBindings()
 }