QtIFW

Calm down the possibility of a buffer overflow.

Still the maximum values do not fit exactly into the double,
though way better than the previous overflow while passing a
large qint64 as qint32 into the function. Fixes some progress
issues I've noticed with very large files that I downloaded.

Change-Id: I7c9ea1fbd8cdd454c3ffd5f952dd0118fdf11c28
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Niels Weber <niels.weber@digia.com>