standard mechanism for recognising and updating out of date default properties

Occasionally, default values of properties in the .jalview_properties will change due to external factors, such as migrations of standard URLs, etc.
A standard mechanism is needed to :
1. recognise when existing user defined property values are likely to be out of date.
2. notify the user if out of date properties exist and offer to replace out of date values.
3. automatically override out of date properties and values if they are now meaningless.