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.

UnitTests/Sources/SettingsScreenViewModelTests.swift

@@ -19,7 +19,8 @@ struct SettingsScreenViewModelTests {
         let userSession = UserSessionMock(.init(clientProxy: ClientProxyMock(.init(userID: ""))))
         viewModel = SettingsScreenViewModel(userSession: userSession,
                                             appSettings: ServiceLocator.shared.settings,
-                                            isBugReportServiceEnabled: true)
+                                            isBugReportServiceEnabled: true,
+                                            isInSecondaryWindow: false)
         context = viewModel.context
     }