added saveStyle capability - Developed for ARPA Piemonte (Dipartimento Tematico Geologia e Dissesto)

Added support into Spatialite provider to save and load styles related to a spatialite layer.
This functionality was already available in PosgreSQl provider. 
This spatialite version it's a simplified version where styles are saved in text field as pure XML encoded UTF8. 
In this way this functionality can be available where Spatialite is available with older version.