Move the PKCS#12 support from QSslSocket to QSslCertificate.

Discussed with Peter and agreed that it's a slightly better fit there.

Change-Id: If8db777336e2273670a23d75d8542b30c07e0d7b
Reviewed-by: Daniel Molkentin <daniel@molkentin.de>
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>