Parse suggested filename from content-disposition

Chromium hasn't parsed content-disposition by the time we get the
download item, so we need to call the parsing manually.

Change-Id: I105d0c6904dd764b368cb774e377a6028c082513
Task-number: QTBUG-48206
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>