xfce4-hdaps-dialogs.c: update to newer combo box API.

xfce4-hdaps-dialogs.c: update to newer combo box API.

The gtk_combo_box_get_active_text() and gtk_combo_box_append_text()
functions have apparently been deprecated since gtk+-2.24. Instead
of calling those functions on a GtkComboBox pointer, we're instead
supposed to call e.g. gtk_combo_box_text_get_active_text() on a
GtkComboBoxText pointer.

Since our "cb_device_name" member was only ever declared as a GtkWidget,
all we have to do is switch everything to the new functions so that they
create, access, and modify the GtkComboBoxText referred to by cb_device_name
instead of a GtkComboBox in the same place.