title: How to detect the change of a GtkStack or GtkStackSwitcher
body: <p>What I'm trying to-do is detect when a user changes the visible child in a GtkStack.</p>
  
  <p>I'm using a GtkStackSwitcher as a visible way to display buttons as per this design:</p>
  
  <p><img src="https://i.imgur.com/srUZ18G.png" alt="pic"></p>
  
  <p>Thus - when a person clicks/toggles one of those buttons I want to perform some processing.</p>
  
  <p>Which event should I connect to achieve this?</p>
  
  <p>I've looked at both <a href="https://lazka.github.io/pgi-docs/#Gtk-3.0/classes/Stack.html#signals" rel="noreferrer">Gtk.Stack</a> and <a href="https://lazka.github.io/pgi-docs/#Gtk-3.0/classes/StackSwitcher.html#signals" rel="noreferrer">Gtk.StackSwitcher</a> but I dont see any obvious signals.  Any thoughts on the best way to proceed?</p>
  
  <p>These don't appear to be standard buttons - so a 'clicked' event I dont think applies in this scenario.</p>
  
  <p>I'm using Ubuntu 14.04 Gtk+3.10, python3</p>