DERBY-534: Support use of the WHEN clause in CREATE TRIGGER statements

Reuse code for dependency checking of the triggered SQL statement for
checking dependencies in the WHEN clause.

Add test to verify that attempts to drop columns referenced in the WHEN
clause detect that the trigger is dependent on the columns.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1530887 13f79535-47bb-0310-9956-ffa450edef68