Add support for spawning windows outside the app

Sources/Adwaita/Model/User Interface/App/App.swift

@@ -41,6 +41,14 @@ extension App {
 
     /// The application's entry point.
     public static func main() {
+        let app = setupApp()
+        app.run()
+    }
+
+    /// Initialize and get the GTUI app.
+    ///
+    /// To run the app, call the ``GTUIApp/run()`` function.
+    public static func setupApp() -> GTUIApp {
         var appInstance = self.init()
         appInstance.app = GTUIApp(appInstance.id) { appInstance }
         GTUIApp.updateHandlers.append { force in
@@ -57,7 +65,7 @@ extension App {
             }
         }
         GTUIApp.appID = appInstance.id
-        appInstance.app.run()
+        return appInstance.app
     }
 
 }

Sources/Adwaita/Model/User Interface/App/GTUIApp.swift

@@ -48,8 +48,16 @@ public class GTUIApp {
     }
 
     /// Run the application.
-    public func run() {
-        let data = ViewStorage.SignalData { self.onActivate() }
+    /// - Parameters:
+    ///     - automaticSetup: Whether the initial windows should be added.
+    ///     - manualSetup: A closure that is executed in the main loop.
+    public func run(automaticSetup: Bool = true, manualSetup: @escaping () -> Void = { }) {
+        let data = ViewStorage.SignalData {
+            if automaticSetup {
+                self.onActivate()
+            }
+            manualSetup()
+        }
         fields["run"] = data
         g_signal_connect_data(
             pointer?.cast(),