Description:
------------
GtkTreeView component complains about UTF-8 encoding when using accented characters. It also doesn't show the part of the string after the accented character.

Reproduce code:
---------------
<?php
$window = new GtkWindow;
$window->set_default_size(200,200);
$window->set_position(GTK::WIN_POS_CENTER);

$list = new GtkTreeView;

$model = new GtkListStore(Gtk::TYPE_STRING);
$list->set_model($model);

$column1 = new GtkTreeViewColumn();

$cell_renderer1 = new GtkCellRendererText();
$column1->pack_start($cell_renderer1, true);

$column1->set_attributes($cell_renderer1, 'text', 0);

$list->append_column($column1);

$data[] = array('Pedro');
$data[] = array('Maria');
$data[] = array('Jos?');
$data[] = array('Autom?vel');
$data[] = array('Jo?o');

foreach ($data as $copa)
{
    $iter = $model->append($copa);
}

$window->add($list);
$window->show_all();
Gtk::Main();
?>

Expected result:
----------------
no warnings, show the right string on GtkTreeView

Actual result:
--------------
warnings, do not show the appropriated string with accented characters.