Fix: Implement color coding for group nodes

CityDoctorParent/Extensions/CityDoctorGUI/src/main/java/de/hft/stuttgart/citydoctor2/gui/tree/node/CityObjectGroupNode.java

 package de.hft.stuttgart.citydoctor2.gui.tree.node;
 
+import de.hft.stuttgart.citydoctor2.check.Checkable;
 import de.hft.stuttgart.citydoctor2.datastructure.CityObject;
+import de.hft.stuttgart.citydoctor2.gui.CheckStatus;
 import de.hft.stuttgart.citydoctor2.gui.Renderer;
 import de.hft.stuttgart.citydoctor2.gui.tree.Renderable;
 import javafx.scene.paint.Color;
@@ -21,7 +23,13 @@ public class CityObjectGroupNode extends Renderable {
 
     @Override
     public void refreshTextColor() {
-        // no color changes
+        if (cityObjects.stream().anyMatch(cityObject -> !cityObject.isValidated())) {
+            setStatus(CheckStatus.NOT_CHECKED);
+        } else if (cityObjects.stream().anyMatch(Checkable::containsAnyError)) {
+            setStatus(CheckStatus.ERROR);
+        } else {
+            setStatus(CheckStatus.OK);
+        }
     }