Please describe the problem:
an unnescessary check in gtk_combo_box_entry_set_text_column prohibits repeated settings of the text-column.

Steps to reproduce:
1. create any GtkComboBoxEntry
2. gtk_combo_box_entry_set_text_column (combo, 0);
3. gtk_combo_box_entry_set_text_column (combo, 0);


Actual results:
an error return in gtk_combo_box_entry_set_text_column's implementation:
  g_return_if_fail (entry_box->priv->text_column == -1);
forbids the second setting of the text-column.

Expected results:
calling the gtk_combo_box_entry_set_text_column the second time should set the
text_column variable to the new value.

Does this happen every time?
happens every time (except the first time of course).

Other information:
the initial value of -1 in text_column does not seem to have any deeper sense except to return an error if called multiple times. So simply removing the g_return_if_fail should work fine. Since one can set the model with `gtk_combo_box_set_model', one should be able to set the text-column, too.