Add a signal for syncing external animation systems

[ChangeLog] QQuickWindow will now emit the afterAnimating() signal
from the gui thread before each scenegraph sync request.

Change-Id: I4897c82f75066238e781455d4fce4fb6bbe2558e
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>