comments on tables

Show them in the list of tables.
Genereate SQL to set the comment.

pglablib/catalog/PgClass.h

@@ -49,6 +49,8 @@ public:
 	std::vector<QString> options;
 	QString viewdef;
 
+    QString description; // from pg_description
+
 	using PgNamespaceObject::PgNamespaceObject;
 
 	QString kindString() const;