If a CheckBox has never been modified, the default value is 0. 
If you want the default value to be 1 (modifying the corresponding propertysheet), the DefaultValue method always return 0, even if 1 is in the propertysheet defined for the field (because of the condition "if value not in (None, ''); value = ob.getProperty(self.key, d=value)". So it was impossible to use the propertysheet to defined a default checked checkbox (the hack was to use tales).
With this new class, it's now possible to use propertysheet to have a checked default value on a CheckBox.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@25166 20353a03-c40f-0410-a6d1-a30d3c3de9de