The class collects all predicate expressions which are in the XSLT model.
Each predicate is declared in the XPath location step.
The location step is bound to the specific schema element or attribute.
The predicate manager keeps the location of predicate in term of
schema elements' path.
The set of predicates is populated from XSLT model. The manager can
be populated entirely or can track small model's changes only.
It produces notification messages about adding, editing or deleting of
the predicates set.
The main intention of the predicate manager is to provide showing of
predicates in the mapper source tree.
author: nk160297
Creates a new predicate and cache it.
Then creates a new predicated node and return it.
This method is intended to be called, when a user creates a new predicate manually.