Prevent unsafe connect by accepting slot in KSDssDownloader constructor

KSDssDownloader commits suicide, so code like:

KSDssDownloader *foo = new KSDssDownloader( ... );
connect( foo, ... );

has potential to fail.

N.B. The better and more correct fix would be to make the one-shot
use-case of KSDssDownloader a static method, just like the one-shot
QTimer. It can internally create a class instance and safely handle
its destruction.