Extract constant for default ID value

VadereState/src/org/vadere/state/attributes/Attributes.java

@@ -17,4 +17,6 @@ package org.vadere.state.attributes;
  * 
  */
 public abstract class Attributes extends DefaultSealable {
+	/** Used for default ID values of some scenario elements. */
+	protected static final int ID_NOT_SET = -1;
 }

VadereState/src/org/vadere/state/attributes/scenario/AttributesSource.java

@@ -14,7 +14,7 @@ public class AttributesSource extends Attributes {
 	public static final String CONSTANT_DISTRIBUTION = ConstantDistribution.class.getName();
 	public static final int NO_MAX_SPAWN_NUMBER_TOTAL = -1;
 
-	private int id = -1;
+	private int id = ID_NOT_SET;
 
 	/** Shape and position. */
 	private VShape shape = null;

VadereState/src/org/vadere/state/attributes/scenario/AttributesStairs.java

@@ -19,7 +19,7 @@ import org.vadere.util.geometry.shapes.VShape;
 public class AttributesStairs extends Attributes {
 
 	private VShape shape = null;
-	private int id = -1;
+	private int id = ID_NOT_SET;
 	private int treadCount = 1;
 	private Vector2D upwardDirection = new Vector2D(1.0, 0.0);
 

VadereState/src/org/vadere/state/attributes/scenario/AttributesTarget.java

@@ -10,7 +10,7 @@ import org.vadere.util.geometry.shapes.VShape;
  */
 public class AttributesTarget extends Attributes {
 
-	private int id = -1;
+	private int id = ID_NOT_SET;
 	/**
 	 * True: elements are removed from the simulation after entering.
 	 * False: the target id is removed from the target id list, but the element remains.