gst: add stream property and notify on new property values

[kohtala]

I need the statistics that are available on the stream object in a gstreamer pipeline. The stream was not accessible from the element.

To access the stream, I add a stream property. Since this property receives value at set_caps when the pipeline is prepared, I need to be notified of the new stream. For that I need to be able to connect to notify::stream signal.

I add the signal emission for both camera and stream properties as they both seem to change value by themselves.