GNOME Data Access 5 manual: GdaClient, GdaConnection and GdaError

## 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.