The type of the function that is used to convert from a value stored in a #GSettings 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.
If @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.
GLib::Variant, Pointer(Void)? -> Bool