Filling a ComboBoxEntry "on demand"

[Luca <elvez84 gmail com>]

Hello, I'm new to this list so please be gentle! :-)

I looked around for some time for a solution, but I still can't find a
way to be notified just before the dropdown list is displayed, since I
need to fill the model with the most up-to-date values in a database.
I tried the "populate_popup" signal of the entry and the
"notify::popup-shown" signal of the combo (available only in GTK+), but
either way, the callbacks don't seem to be called at the right moment.
Also, I need the application to run on Windows, so at the moment I can't use features not present in version 2.8.8.
Thank you very much for any help, 

Luca