Doc: the implicit size of ProgressBarStyle's background must be set.

Change-Id: I54f160ed3fb875b694909e6d9eddcb500566422c
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
use private linkage where possible

Change-Id: I693f8469b8bec8e80d65d69122c01d0273a38b3d
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>