Perform the persistence asynchronously to not block the main (UI) thread

This avoids "your app is not responding" errors, while the git command is waiting for
password input to load the gpg key for commit signing.

This also reverts the fix from 7cc0463:
Calling System#exit immediately after Platform#exit prevents Application#stop from
being called. And we always exited with 0. Now the exit status is determined by
DesignerStarter#launchGui and used correctly. Platform#exit triggers the shutdown
of JavaFx. After that is done execution continues in DesignerStarter after
Application#launch.

Author: adangel

src/main/java/net/sourceforge/pmd/util/fxdesigner/Designer.java

@@ -21,6 +21,7 @@
 import javafx.application.Application;
 import javafx.application.Platform;
 import javafx.collections.ObservableList;
+import javafx.concurrent.Task;
 import javafx.fxml.FXMLLoader;
 import javafx.scene.Parent;
 import javafx.scene.Scene;
@@ -97,11 +98,18 @@ public void start(Stage stage, DesignerRoot owner) throws IOException {
         ));
 
         stage.setOnCloseRequest(e -> {
-            owner.getService(DesignerRoot.PERSISTENCE_MANAGER).persistSettings(mainController);
-            Platform.exit();
-            // VM sometimes fails to exit for no apparent reason
-            // all our threads are killed so it's not our fault
-            System.exit(0);
+            Task<Void> closerTask = new Task<Void>() {
+                @Override
+                protected Void call() throws Exception {
+                    owner.getService(DesignerRoot.PERSISTENCE_MANAGER).persistSettings(mainController);
+                    return null;
+                }
+            };
+            closerTask.setOnSucceeded(event -> {
+                Platform.exit();
+            });
+            new Thread(closerTask).start();
+            e.consume(); // don't close the window yet, will be closed by Platform#exit
         });
 
         Parent root = loader.load();