Added a psuedo "subclassable propertysheet class interface" to prevent

us from inteheriting from propertysheet classes that weren't meant to
be subclassed.  This fixes a problem with creating property sheets for
subclasses of bilt-in classes like Folder.

lib/python/ZClasses/Property.py

@@ -386,6 +386,7 @@ def klass_sequence(klass,attr,result=None):
 class ZInstanceSheets(OFS.PropertySheets.PropertySheets, Globals.Persistent):
     " "
     __propset_attrs__=()    
+    _implements_the_notional_subclassable_propertysheet_class_interface=1
 
     def __propsets__(self):
         propsets=ZInstanceSheets.inheritedAttribute('__propsets__')(self)