Set "entry-text-column" on our combo boxes.

Set "entry-text-column" on our combo boxes.

When we provide an editable text box with a drop-down list of useful
preset values, such as the one full of character sets in the
Translation panel, we implement it on GTK 2.4+ as a GtkComboBox
pointing at a two-column GtkListStore, in which the second column is
the actual text (the first being a numeric id). Therefore, we need to
set the "entry-text-column" property to tell GtkComboBox which of
those columns to look in for the value corresponding to the edit-box
text.

Thanks to Robert de Bath for spotting the problem and tracing it as
far as commit 5fa22495c. That commit replaced a widget construction
call via gtk_combo_box_entry_new_with_model() with one using the newer
gtk_combo_box_new_with_model_and_entry(), overlooking the fact that
the former provided the text column number as a parameter, and the
latter didn't.