Implement and use window registry

In an application, you may have one or more video renderers. You can have native widgets, on OS X something NSView based, that you can render into to embed the video rendering into your GUI.

You would do this by calling, e.g.:
```
owr_window_registry_register(owr_window_registry(), "self-view", selfView);
```
And then when you create your video renderer, pass the tag to the constructor, e.g.:
```
video_renderer = owr_video_renderer_new("self-view");
```

Then when you set the source of the renderer, the internals of OwrVideoRenderer will look up the window handle and correctly set it on the glimagesink in order to embed the video into the widget.
Shouldn't this be a singleton, so that it is never finalized at all?