Several changes on schema files related to comment command formatting.

Removed the attribute ParsersAttributes::DIF_SQL.

Created custom implementation for getSignature() for Aggregate, Cast,
Constraint, Index, Op. Class, Op. Family, Rule, Trigger

Removed commented code related to conditional signature usage.

Added a schema file for ALTER RENAME TO command.
Merge branch 'schema-files-improvements' of https://github.com/pgmodeler/pgmodeler into schema-files-improvements