This is an implementation of new interface for DDL operation named GdaDdlModifiable. All objects, e.g. GdaDbTable, GdaDbView implement it. It allows add and remove the corresponding objects from the DB as well as rename them if the provider supports the operation. The corresponding test was written for SQLite3 provider. This MR affects only GdaDb module. The corresponding sections were added to the documentation. Other documentation bugs were fixed too.