Add additional constructor to ctkCallback

2 constructors is usually better:

ctkCallback(QObject* parentObject = 0);
ctkCallback(void (*callback)(), QObject* parentObject = 0);

rather than:

ctkCallback(void (*callback)() = 0, QObject * parentObject = 0);

Libs/Core/ctkCallback.cpp

@@ -25,6 +25,12 @@
 // ctkCallback methods
 
 // --------------------------------------------------------------------------
+ctkCallback::ctkCallback(QObject * parentObject) : QObject(parentObject)
+{
+  this->setCallback(0);
+}
+
+// --------------------------------------------------------------------------
 ctkCallback::ctkCallback(void (*callback)(), QObject * parentObject) : QObject(parentObject)
 {
   this->setCallback(callback);

Libs/Core/ctkCallback.h

@@ -33,7 +33,8 @@ class CTK_CORE_EXPORT ctkCallback : public QObject
   Q_OBJECT
 public:
 
-  ctkCallback(void (*callback)() = 0, QObject * parentObject = 0);
+  ctkCallback(QObject * parentObject = 0);
+  ctkCallback(void (*callback)(), QObject * parentObject = 0);
   virtual ~ctkCallback();
   
   void (*callback())()const;