owr_video_renderer: add sink property

Make the video sink configurable at renderer creation time for further
flexibility. In some cases the default glimagesink can't be used. The
guintptr value should point to a valid GStreamer element.
Right, so just don't add it to the constructor. We can take care of removing the window handle from the constructor later.

As a note, I don't know if we will land this because I will implement a different approach very soon. We need to have some way to map from a tag in a page to a window handle or element or so. We're going to implement a wrapped hash table with API to register tag and window handle or element pointer.