Small API-Change: Add signal to write and read custom data to the maplayer in a .qgs-file

New signals:
 * void QgsProject::writeMapLayer( QgsMapLayer*, QDomElement&,  QDomDocument& )
 * void QgsProject::readMapLayer( QgsMapLayer*, const QDomElement& )

Changed method signature:
( Due to names being not really distinguishable from readXml / writeXml)
 * bool QgsMapLayer::readXML( const QDomNode& )
   => bool QgsMapLayer::readLayerXML( const QDomElement& )
 * bool QgsMapLayer::writeXML( QDomNode&, QDomDocument& )
   => bool QgsMapLayer::writeLayerXML( QDomElement&, QDomDocument& )