GdaClient, GdaConnection and GdaError

The GdaClient usage has remained unchanged except for some small API alterations and a specific API has been defined to create and destroy databases.

The GdaError class has been renamed to GdaConnectionEvent and can now convey not only errors but also other kind of events which have happened on a connection

The GdaConnection usage has remained by far unchanged, but some of its API have had some alterations (finding the equivalent is trivial). The most noticeable change being that the gda_connection_escape_string() has been removed because string escaping is now done by a GdaDataHandler object which each GdaServerProvider provides for each data type.