Update copyright headers

Qt copyrights are now in The Qt Company, so we could update the source
code headers accordingly. In the same go we should also fix the links to
point to qt.io.

Change-Id: Ieb3be34b0d4061fb8dc886991528951f59dbd67e
Reviewed-by: Tuukka Turunen <tuukka.turunen@digia.com>
Reviewed-by: Iikka Eklund <iikka.eklund@theqtcompany.com>