REINITIALIZE-INSTANCE support for class ANNOTATABLE

The class ANNOTATABLE has now additional support for REINITIALIZE-INSTANCE;
in particular, the OBJECT-PLIST property can now be preserved across re-
initialisations.

Whether the property list is preserved or not depends on the presence (and
value) of the new PRESERVE-OBJECT-PLIST initarg (note, that this is intentionally
not a keyword). If the argument is supplied with a value of true, then an
existing property list is preserved; if a different property list is also
supplied (via the OBJECT-PLIST initarg), then its value is merged with the
old property list (with values from the new one taking precedence over old
values).