Added classes for Notifications (Sound playback use case)

Added classes for Notifications (Sound playback use case)

The main class is QtCamNotifications which emits various signals when
image capture starts and ends and video recording starts and ends.

QML bindings consist of a Notifications interface and a notifications property
for Camera element.

We cannot use simple signals because at least the image capture start notification
gets emitted from another thread for performance reasons.