owr_video_renderer: add sink property

Make the video sink configurable for further flexibility. In some
cases the default glimagesink can't be used. The new GObject property
should point to a valid GStreamer element.
owr_audio_renderer: add sink property

Make the audio sink configurable for further flexibility. In some
cases the default sink can't be used or the application needs direct
access to it for volume control purposes or even position queries.
The new GObject property should point to a valid GStreamer element.