This is mainly intended to fill the proplist of the definition and replace
the defcore, but other one time initialisation could also be done at that
point.
A Proplist is a map of C4Strings to C4Values which can chain lookups
to "Prototype" Proplists. This can be used to implement inheritance.
Proplists can be stored in C4Values.
C4Object now inherits from C4Proplist, C4Values can be converted between
those two.