eolian: allow extending eolian-generated classes from within C

eolian: allow extending eolian-generated classes from within C

If you define either the macro MY_CLASS_EXTRA_OPS for normal
methods/properties or MY_CLASS_EXTRA_CLASS_OPS for class methods
or properties, which contains a comma-delimited list of ops defs
(i.e. EFL_OBJECT_OP_FUNC(...), ...) right before including the
generated my_class.eo.c file, the definitions from these will
be included in the actual class. This can be used to override
certain things in a class internally without exposing it to
Eolian, or for testing/debugging.