Add callbacks for camera enabled/disabled

[jimboca]

When a camera is enabled or disabled in the app no callback is executed.

[chao-camect]

We do have events like "alert_disabled", "alert_enabled", "camera_offline", "camera_online".
When you disable a camera, an "camera_offline" event will be sent after about 3 minute (default).

[jimboca]

@chao-camect somehow I missed your response to this. These are nice, but would also be good to have callbacks for camera being disabled/enabled and streaming/notstreaming. Also camera online/offline is not sent with data returned by list_cameras