Introduced a few more signals to QQuickWindow.

Also marked the new afterAnimation() signal as \since 5.3 and
introduced proper revisioning on the new signals.

[Changelog][QtQuick] Added QQuickWindow::afterSynchronizing(),
openglContextCreated(), sceneGraphAboutToStop(). Useful for
deeper integration with the rendering.

Change-Id: I5532b310506c2432325595e55ef9307b8934abee
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>