Feat: Add IndeterminateSpinnerDialog

Adds a generalized version of the 3D-Renderer LoadingDialog to improve
UX by showing that CityDoctor is currently busy doing something in the
background of indeterminate length, e.g. opening a file or waiting for
the Schematron thread to finish.

CityDoctorParent/Extensions/CityDoctorGUI/src/main/java/de/hft/stuttgart/citydoctor2/gui/IndeterminateSpinnerDialog.java

+package de.hft.stuttgart.citydoctor2.gui;
+
+import javafx.fxml.FXML;
+import javafx.fxml.FXMLLoader;
+import javafx.scene.Scene;
+import javafx.scene.control.Label;
+import javafx.scene.layout.VBox;
+import javafx.stage.Modality;
+import javafx.stage.Stage;
+import javafx.stage.StageStyle;
+import javafx.stage.Window;
+
+import java.io.IOException;
+
+public class IndeterminateSpinnerDialog {
+
+
+    private Stage stage;
+
+    @FXML
+    private Label messageLbl;
+
+    public IndeterminateSpinnerDialog(Window parent, String message) throws IOException {
+        FXMLLoader loader = new FXMLLoader(LoadingInfoDialog.class.getResource("IndeterminateSpinnerDialog.fxml"));
+        loader.setController(this);
+        VBox box = loader.load();
+
+        stage = new Stage(StageStyle.UNDECORATED);
+        Scene scene = new Scene(box);
+        scene.setFill(null);
+        stage.setScene(scene);
+        stage.initOwner(parent);
+        stage.initModality(Modality.APPLICATION_MODAL);
+        messageLbl.setText(message);
+    }
+
+    public void changeMessage(String message){
+        messageLbl.setText(message);
+    }
+
+
+    public void show() {
+        stage.show();
+    }
+
+    public void hide() {
+        stage.hide();
+    }
+
+}
+
+

CityDoctorParent/Extensions/CityDoctorGUI/src/main/resources/de/hft/stuttgart/citydoctor2/gui/IndeterminateSpinnerDialog.fxml

+<?xml version="1.0" encoding="UTF-8"?>
+
+<?import javafx.geometry.Insets?>
+<?import javafx.scene.control.Label?>
+<?import javafx.scene.control.ProgressIndicator?>
+<?import javafx.scene.layout.VBox?>
+<?import javafx.scene.text.Font?>
+
+<VBox alignment="CENTER" maxHeight="-Infinity" maxWidth="-Infinity" spacing="10.0" xmlns="http://javafx.com/javafx/22" xmlns:fx="http://javafx.com/fxml/1">
+   <children>
+      <Label fx:id="messageLbl" text="Message">
+         <font>
+            <Font size="14.0" />
+         </font>
+      </Label>
+      <ProgressIndicator />
+   </children>
+   <padding>
+      <Insets bottom="20.0" left="20.0" right="20.0" top="20.0" />
+   </padding>
+</VBox>