fix: avoid create menu actions everytime when trigger context menu.

fix the minor memory leak issue, also bring some possibility to
implement custom keybinding.

And oops I also did another feature in this commit..
Now we are able to set mouse wheel behavior in config dialog.

app/settingsdialog.h

@@ -20,6 +20,7 @@ public slots:
 private:
     QCheckBox * m_stayOnTop = nullptr;
     QComboBox * m_doubleClickBehavior = nullptr;
+    QComboBox * m_mouseWheelBehavior = nullptr;
 };
 
 #endif // SETTINGSDIALOG_H