A very general validator strategy that essentially returns true if the current state is
considered to be legal by the instance of the validator.
author: Jesse since: 1.1.0
isValid(EditToolHandler handler, MapMouseEvent event, EventType type) Returns null if the validator considers the state to be "legal" for the new event or
a string which is the human readable message describing the problem.
Parameters: handler - the handler to use for obtaining the state Parameters: event - the event that just occurred.
Returns null if the validator considers the state to be "legal" for the new event or
a string which is the human readable message describing the problem.
Parameters: handler - the handler to use for obtaining the state Parameters: event - the event that just occurred. Parameters: type - they type of event null if the validator considers the state to be "legal" for the new event ora string which is the human readable message describing the problem.