We can not add a new virtual method without breaking ABI on some
platforms, instead we need to use a setter.

The API now uses a request object, and a separate signal for canceling,
since canceling can not be rejected.

Change-Id: If8069c343e86926293c30e8de179bf4e3cbd5886
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>