It's a composite widget (an HBox) and it has an entry and a list.
You can use it like an entry and to track changes you should connect
to the "changed" signal of combo->entry.
The list can contain anything, but you should supply a function
if you want to use widgets other than GtkLabels to get a string out
of your widgets (an example is provided for a two-column list).
The value in the entry can be limited to those in the list
using gtk_combo_set_flag(combo, GTK_COMBO_IN_LIST_VALUE).
It would be useful to add completion support in the entry.
I haven't found a way to set the size correctly for the popup
window (it's incorrect for lists with few items).
