label: Don't use the selection too easily

When we are re-setting the same text for internal reasons
(e.g. when applying the mnemonics-visible change upon Alt press),
we should not needlessly loos the selection.

https://bugzilla.gnome.org/show_bug.cgi?id=671588

gtk/gtklabel.c

@@ -1982,11 +1982,14 @@ gtk_label_set_text_internal (GtkLabel *label,
                              gchar    *str)
 {
   GtkLabelPrivate *priv = label->priv;
+  gboolean text_changed;
+
+  text_changed = g_strcmp0 (priv->text, str) != 0;
 
   g_free (priv->text);
-
   priv->text = str;
 
+  if (text_changed)
     gtk_label_select_region_index (label, 0, 0);
 }