How to use GtkComboBoxText

1. create
2. add some values, with
3. get value with
GtkComboBoxTextwith a entry;
2. add some values, with
GtkComboBoxText::append,
GtkComboBoxText::append_text,
GtkComboBoxText::insertand anothers methods;
3. get value with
GtkComboBoxText::get_active_text;