Add QAccessible::ActivationObserver

Makes it possible to follow changes in activation an deactivation of a11y.
Needed for WebEngine to know when to activate a11y.

Change-Id: Ia264a76974224d1baad3e88c34a4b8a9f1a3695d
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>
Fix msaa in QOpenGLWidget

Take the correct number of samples from the widget's context, not the
tlw's context.

The original implementation did the blit only after paintGL(). This is
not sufficient for applications that override paintEvent() and do QPainter
calls in there (e.g. the 2dpainting example). Therefore the approach is
changed to perform the resolving of the samples in a function that is
invoked by QWidgetPrivate every time a paint event is sent.

Change-Id: Iae0b2c30f6070ec75201339a848854e4582a9c0c
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>