pgsql 7.5 frontend changes wrapup

With 7.5 feature freeze coming nearer, administrative interface
developers probably would like a list what new features they should
support, i.e. which ddl features where added. Here's a list of relevant
changes as far as I extracted them, please complete.

- $ Quoting
- TABLESPACE
- ALTER TABLE ALTER COLUMN TYPE
- COMMENT ON CAST/CONVERSION/LANGUAGE

Regards,
Andreas