how to get entry from GtkComboBoxEntry?

[Vlasov Vitaly <vnigtha gmail com>]

Hello list.

I wanna use some ComboBox/Entry widget:
It should contain tree model with language names and hidden ISO 639-1
codes, for example "English"("en") "German"("de") "Russian"("ru") or
other. And, in addition, it should allow to user write ISO 639-1
language code from keyboard.

So, with tree i can get selected value and respectively get lang code
from tree.
When in ComboBox is not selected values, it could mean: 1) User set it
from keyboard 2) Entry is empty.

It is possible to create ComboBox with tree model and separate entry,
but i don't like that way.

How to check, whether the user has entered a value from the keyboard or
entry is empty?
May be i should use other Entry/ComboBox widget? I used before
GtkEntryCompletion. It work's, but it uncomfortable with two-letter ISO
codes.

Now i use gtk-2.0, but later will switch to 3.0. As i can see, in
gtk-3.0 GtkComboBox and GtkComboxEntry is same thing. But i still can't
get entry widget from gtk-3.0 ComboBox.