XLS Decision tables: Add support for Condition column DRL fragments

Condition columns in XLS Decision Tables have a row for Object type and a row for code "snippet". The code snippet is taken as predicates within the Object type; for example:-

CONDITION    | CONDITION
----------------------------
           MyFact
----------------------------
field1 ==    | field2 ==
----------------------------
a            | b

This is rather limiting (and certainly not conducive to decision table round-tripping). Ideally we need to support:-

CONDITION
----------------------------------------------
MyFact( field1 == $param1, field2 == $param2 )
----------------------------------------------

----------------------------------------------
a,b

Even more ideally it'd be good to be able to register "column handlers" so this can be extended easily.