Finds the proper context in which to store a property.
If we have installed a property entry handler via PropChunk()
and such a property chunk (for example id is "CMAP" and type is "ILBM"
inside a form, then the storedproperty will be stored in the enclosing
FORM chink. If the chunk was inside a PROP chunk inside a LIST, then
the storedproperty would be installed in the LIST context.
INPUTS
iff
pointer to IFFHandle struct.
RESULT
cn - pointer to contextnode where the property might be installed, or
NULL if no such context exists.