temporary adding this file so it become versioned, but we'll remove it again because apparently it's not so easy to trigger missing notifications, so we're unable to test the missing notification callbacks
removed test because it doesnt work: it's not possible (?) for the client to invoke missing notifications
Added or improved support for the following events: ConnectionStatusChanged, SubscriptionStatusChanged, NotificationsMissing and KeepAlive.

On the C++ side, the following methods were added to uafc::ClientInterface:
 - connectionStatusChanged
 - subscriptionStatusChanged
 - notificationsMissing

On the Python side, the following methods were added to pyuaf.client.Client:
 - connectionStatusChanged and registerConnectionStatusCallback
 - subscriptionStatusChanged and registerSubscriptionStatusCallback
 - notificationsMissing and registerNotificationsMissingCallback
 - registerKeepAliveCallback (keepAliveReceived existed already)
Added unit tests for the previously added functionality