title: C++ - How to display an icon in a dialog in gtkmm
body: <p>I have several Dialog in my program that tells the users various things, from errors, to confirmation.</p>
  
  <p>The sample in the documentation tells me to use this to have an icon set depending on the dialog (warning, information, question mark...):</p>
  
  <pre><code>Gtk::MessageDialog dialog(*this, "This is a QUESTION MessageDialog",
      false /* use_markup */, Gtk::MESSAGE_QUESTION,
      Gtk::BUTTONS_OK_CANCEL);
  </code></pre>
  
  <p>However, I do not get any icon in the dialog, I am only able to change the icon in the top bar, using this :</p>
  
  <pre><code>Gtk::MessageDialog dialog(*this,~     
      "Please select something first.",
      false,                        
      Gtk::MESSAGE_ERROR,           
      Gtk::BUTTONS_OK);             
  dialog.set_icon_name("dialog-error"); 
  dialog.run();                         
  </code></pre>
  
  <p>I get no error, no warning, this compile and execute just fine, but the resulting message box doesn't have any icon in it. How can I get an icon to display properly INSIDE my dialog box ? I also tried the other MESSAGE_* available, without success.</p>