Recompute label contents when the buffer changes

Also reset cursor positions to avoid assertions.
This fixes bug 612505, patch by Tristan Van Berkom.
(cherry picked from commit 233281fd39)

gtk/gtkentry.c

@@ -6667,6 +6667,9 @@ gtk_entry_set_buffer (GtkEntry       *entry,
   g_object_notify (obj, "invisible-char");
   g_object_notify (obj, "invisible-char-set");
   g_object_thaw_notify (obj);
+
+  gtk_editable_set_position (GTK_EDITABLE (entry), 0);
+  gtk_entry_recompute (entry);
 }