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.

[EmmanuelP]

Hi,

I have created and applied to aravis-0-8 another pull request splitting your changes here: #928

The changes related to camera property notify signal is not included, as the logic for the detection of a camera change seems strange. You are comparing 2 pointers with one of them is freed. There is nothing preventing the new allocated camera object is not at the same place as the one freed.

Please create a new pull request with only the notify:camera signal code, targeting aravis-0-8.