Feat: Add TopologyErrorNode class

Adds a general node class for TopologyErrors, which allows custom
rendering/highlighting implementations for each error with a strategy
pattern.

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

+package de.hft.stuttgart.citydoctor2.gui.tree.node;
+
+import de.hft.stuttgart.citydoctor2.gui.Renderer;
+import de.hft.stuttgart.citydoctor2.gui.tree.Renderable;
+
+public class TopologyErrorNode extends Renderable {
+
+    private final RenderStrategy strategy;
+    private final String text;
+
+    public TopologyErrorNode(String text, RenderStrategy strategy) {
+        this.strategy = strategy;
+        this.text = text;
+    }
+
+
+    @Override
+    public void refreshTextColor() {}
+
+    @Override
+    public String getText() {
+        return text;
+    }
+
+    @Override
+    public void visit(Renderer renderer) {
+        strategy.render(renderer);
+    }
+
+    /**
+     * Strategy-pattern Interface for rendering/highlighting of the geometries in a TopologyError
+     */
+    public interface RenderStrategy {
+        void render(Renderer renderer);
+    }
+}