QCryptographicHash: allow to hash the content of a QIODevice

QCryptographicHash: allow to hash the content of a QIODevice

This adds a new function (and tests) to give the possibility of doing a
QCryptographicHash of a QIODevice, like a QFile or whatever people
needs.

It is a quite handy overload in many cases.

Change-Id: I22fd272f05571844641b3daefcc6746be4e5c7c3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>