Ask to save the current layout when closing shuffleboard

Fixes wpilibsuite#173

api/src/main/resources/edu/wpi/first/shuffleboard/api/base.css

@@ -235,3 +235,11 @@
     /* Bring the content to all edges */
     -fx-padding: 0; /* 10 0 0 10 */
 }
+
+.dialog-pane .header-panel {
+    -fx-background-color: -swatch-500;
+}
+
+.dialog-pane .header-panel .label {
+    -fx-text-fill: white;
+}

app/src/main/java/edu/wpi/first/shuffleboard/app/Shuffleboard.java

@@ -28,6 +28,8 @@
 import javafx.application.Platform;
 import javafx.fxml.FXMLLoader;
 import javafx.scene.Scene;
+import javafx.scene.control.Alert;
+import javafx.scene.control.ButtonType;
 import javafx.scene.layout.Pane;
 import javafx.stage.Screen;
 import javafx.stage.Stage;
@@ -90,6 +92,25 @@ public void start(Stage primaryStage) throws IOException {
     primaryStage.setMinHeight(480);
     primaryStage.setWidth(Screen.getPrimary().getVisualBounds().getWidth());
     primaryStage.setHeight(Screen.getPrimary().getVisualBounds().getHeight());
+    primaryStage.setOnCloseRequest(closeEvent -> {
+      Alert alert = new Alert(Alert.AlertType.CONFIRMATION);
+      alert.getDialogPane().getScene().getStylesheets().setAll(mainPane.getStylesheets());
+      alert.getButtonTypes().setAll(ButtonType.YES, ButtonType.NO, ButtonType.CANCEL);
+      alert.getDialogPane().setHeaderText("Save the current layout?");
+      alert.showAndWait().ifPresent(bt -> {
+        if (bt == ButtonType.YES) {
+          try {
+            mainWindowController.save();
+          } catch (IOException ex) {
+            logger.log(Level.WARNING, "Could not save the layout", ex);
+          }
+        } else if (bt == ButtonType.CANCEL) {
+          // cancel the close request by consuming the event
+          closeEvent.consume();
+        }
+        // Don't need to check for NO because it just lets the window close normally
+      });
+    });
     primaryStage.show();
     Time.setStartTime(Time.now());
   }