Add missing \since tags for new API introduced by Qt 5.4.

Change-Id: Ie51d6e549d80baae57864d4e0d43a1c58897f149
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>