Fix simplebrowser crash on exit while downloading

On webengine profile destruction all download
items in progress are canceled. This triggered
stateChanged of QWebEngineDownloadItem which
called lambda function on already deleted object.

Do not use lambda functions in this case.

Change-Id: Ia1d359fe47204baa51f1380d1c3547c28550bc28
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Viktor Engelmann <viktor.engelmann@qt.io>