title: C++ QQuickItem: How to trigger a function on item size change
body: <p>I have a QQuickItem in C++ which is instantiated in a QML application. I want to trigger a custom function on the c++ side if the width property of my QQuickItem changes.</p>
  
  <p>In QML I would do onWidthChanged: { my code}. How can I do this on the c++ side?</p>