OAK-822: TypeEditor: Missing validation for mandatory items

Improved handling of mandatory items:
Now the presence of *all* mandatory items is only checked
when a new node is being added. Afterwards the "mandatoriness"
of items is only checked when they are being removed.
Error codes documented in http://wiki.apache.org/jackrabbit/OakErrorCodes.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1483779 13f79535-47bb-0310-9956-ffa450edef68