HR fix review: 229648 - IDE locked when creating web service client

229648 - IDE locked when creating web service client
https://netbeans.org/bugzilla/show_bug.cgi?id=229648

Problem: random deadlock

Fix: EjbContainerChildren was too eager and was accessing EJB metamodel 
from constructor to add a listener. I moved the code and the listener is 
now added only after node was expanded and its children are being 
created for the first time

Risk: low I think; the fix reuses existing already synchronized call to 
metamodel to add the listener

Diff: http://hg.netbeans.org/web-main/rev/e40617d963be

Thanks,
-David