Add changelog for 5.3.0

Change-Id: I99fadbd5dff0a4ec5e6abd5b8a6b5b19cbfcace7
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>