title: Write custom widget with GTK3
body: <p>I am trying to find the simplest example of a custom widget being written for Gtk-3.</p>
  
  <p>So far the best thing I've found is <a href="http://www.pygtk.org/articles/writing-a-custom-widget-using-pygtk/writing-a-custom-widget-using-pygtk.htm" rel="noreferrer">this</a> (using PyGTK), but it seems to be targeted to Gtk-2.</p>
  
  <p>BTW: I don't care the language it is written in, but if we can avoid C++, much better!</p>