implement keyboard zooming into node view

app/widget/nodeview/nodeview.h

@@ -74,6 +74,10 @@ public:
 
   void SetColorLabel(int index);
 
+  void ZoomIn();
+
+  void ZoomOut();
+
 signals:
   void NodesSelected(const QVector<Node*>& nodes);
 
@@ -102,6 +106,10 @@ private:
   void ConnectSelectionChangedSignal();
   void DisconnectSelectionChangedSignal();
 
+  void ZoomIntoCursorPosition(double multiplier, const QPointF &cursor_pos);
+
+  void ZoomFromKeyboard(double multiplier);
+
   NodeGraph* graph_;
 
   struct AttachedItem {
@@ -135,6 +143,8 @@ private:
 
   double scale_;
 
+  static const double kMinimumScale;
+
 private slots:
   /**
    * @brief Receiver for when the scene's selected items change