Fix iconUrl getter and changed signal

We should only emit iconUrlChanged when iconUrl changes, and the return
value of iconUrl should match the value of the last emitted change.

Change-Id: I70f4d6a11b96d83278dddc4d85e1c78ca82bf5cb
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>