always set a background for docking area

dockingcore/src/main/java/fr/soleil/docking/ADockingManager.java

 package fr.soleil.docking;
 
+import java.awt.Color;
 import java.beans.PropertyChangeEvent;
 import java.beans.PropertyChangeListener;
 import java.io.File;
@@ -71,9 +72,12 @@ public abstract class ADockingManager implements PropertyChangeListener {
     /**
      * Creates a new docking area
      * 
+     * @param areaBackground The preferred area background. May be <code>null</code>, in which case this
+     *            {@link ADockingManager} chooses the {@link Color} to use
+     * 
      * @return A {@link JComponent}
      */
-    public abstract JComponent createNewDockingArea();
+    public abstract JComponent createNewDockingArea(Color areaBackground);
 
     /**
      * Returns the perspective factory