Add API to write and read custom data to the layer in a .qgs-file.
New signals:
 * void QgsProject::writeMapLayer( QgsMapLayer*, QDomElement&,  QDomDocument& )
 * void QgsProject::readMapLayer( QgsMapLayer*, const QDomElement& )

Changed method signature:
 * bool QgsMapLayer::readXML( const QDomNode& )
   => bool QgsMapLayer::readLayerXML( const QDomElement& )
 * bool QgsMapLayer::writeXML( QDomNode&, QDomDocument& )
   => bool QgsMapLayer::writeLayerXML( QDomElement&, QDomDocument& )