#34 DbCreator: support expression default values such as Now() or Date()

[Thomas Mueller]

A column default read from an Access database (ADOX Default property) can be an expression such as Now() or Date() rather than a literal. The DbCreator writers always quote the default (DEFAULT '%s' in u_dzDbCreatorCreateMsSql and u_dzDbCreatorCreateOracle), so such an expression ends up as a string literal in the DDL.

Expression defaults should be recognised, kept apart from literal ones in the description (and the XML), and written unquoted, translated to each target database's syntax (e.g. GETDATE() for MS SQL, SYSDATE for Oracle).

Follow-up to [bugs:#66].