Added virtual destructor to all interfaces

src/control/ZoomControl.cpp

@@ -2,6 +2,8 @@
 
 const double zoomStep = 0.04;
 
+ZoomListener::~ZoomListener() { }
+
 ZoomControl::ZoomControl()
 {
 	XOJ_INIT_TYPE(ZoomControl);

src/control/ZoomControl.h

@@ -26,6 +26,8 @@ class ZoomListener
 public:
 	virtual void zoomChanged(double lastZoom) = 0;
 	virtual void zoomRangeValuesChanged();
+
+	virtual ~ZoomListener();
 };
 
 class ZoomControl

src/undo/UndoRedoHandler.cpp

@@ -49,6 +49,8 @@ void printUndoList(GList* list)
 #define PRINTCONTENTS()
 #endif //UNDO_TRACE
 
+UndoRedoListener::~UndoRedoListener() { }
+
 UndoRedoHandler::UndoRedoHandler(Control* control)
 {
 	XOJ_INIT_TYPE(UndoRedoHandler);

src/undo/UndoRedoHandler.h

@@ -21,6 +21,8 @@ class UndoRedoListener
 public:
 	virtual void undoRedoChanged() = 0;
 	virtual void undoRedoPageChanged(PageRef page) = 0;
+
+	virtual ~UndoRedoListener();
 };
 
 class UndoRedoHandler