The type of the function that is used to convert from a value stored in a Settings to a value that is useful to the application.
If the value is successfully mapped, the result should be stored at
result and true returned. If
mapping fails (for example, if
value is not in the right format) then false should be returned.
value is null then it means that the mapping function is being given a "last chance" to
successfully return a valid value. true must be returned in this case.
the Variant to map, or null
the result of the mapping
the user data that was passed to get_mapped
true if the conversion succeeded, false in case of an error