structure: Add a .keys() method and implement __str__
structure: Add a .keys() method and implement __str__

We are making it behave like a dict, so we should provide the
same kind of utilities.

pexip/gst-python,GStreamer/gst-python,pexip/gst-python,GStreamer/gst-python,GStreamer/gst-python,pexip/gst-python