Add GML3 to QgsOgcUtils

After the commit 969df016bcdbd1ead8a347b7a5d55c8b36db183a Moved GML import/export to a new class: QgsOgcUtils, I decided to add GML3.
I updated QgsOgcUtils, QGIS WFS Server, and the Python Iterface and I added unit test for GML3.