Create interface for manage the logic of specific elements

src/main/java/model/elements/ElementHandler.java

+package model.elements;
+
+import model.ModelElement;
+import util.Position;
+
+public interface ElementHandler {
+    void addElement(Position position);
+    void removeElement(Position position);
+    boolean hasElement(Position position);
+    ModelElement getModelElement();
+}