Remove behaviors listed in stackfiles when closing a ui stack with destroystack set to true

[trevordevore]

When closing a ui stack which has destroyStack set to true Levure should remove all of the behaviors from memory as well. The behaviors should only be removed, however, if no other open stacks are referencing the behavior.

LiveCode 9 added the revbehaviourses function. It takes a reference to an object and returns an array list of object long IDs for objects that are using a behavior.

PR for revBehaviorUses:
livecode/livecode#5788