title: GTK - How to know that the dropdown list of the GtkComboBoxText gets shown
body: <p>A GtkComboBoxText consists of a text entry, a dropdown list and a button to show the dropdown list.</p>
  
  <p>I want to populate the contents of the drop down box of a GtkComboBoxText when the user clicks on the dropdown button. This is because the drop down box needs to be repopulated with the data.</p>
  
  <p>I found that pressing alt-down in the text entry generates the "popup" signal and opens the drop down box, but I can't find the right signal which one which matches the even when the dropdown button is pressed.</p>
  
  <p>What would be the right signal I need to connect to?
  Or is there another way to repopulate the contents of the dropdown list when it gets displayed?</p>
  
  <p>Thanks in advance, Edwin</p>