Added custom validity for attribute items.
This PR adds custom validity for attribute items with an API much like custom relevancy, where users are able to assign a function with custom logic to determine its respective validity.
- :smtk:
Attribute::isValid
- method to call to see if an attribute is valid- :smtk:
Item::isValid
- method to call to see if an item is valid- :smtk:
Item::defaultIsValid
- default validity method used by :smtk:Attribute::isValid
when no custom validity function has been specified- :smtk:
Item::setCustomIsValid
- method used to set a custom validity function- :smtk:
Item::customIsValid