Added: Qp::lastError, which always returns the last global error.
Added: Transactions for schema adjustments and UPDATE and DELETE

Added two methods:
bool beginTransaction();
CommitResult commitOrRollbackTransaction();

The first is trivial. The second uses QSqlDatabase::commit or rollback, based upon whether Qp::lastError returns a valid error. Qp::lastError takes also QSqlDatabase::lastError into account, so that every SQL error should be registered.

The underlying database should now always be consistent.