OwrWindowRegistry: Implement registry for mapping tag to window handle

This is just a hash table wrapper with API. The OpenWebRTC API user will
be expected to register tags against window handles before renderers are
created. When the renderer is created, a tag is passed to its
constructor. This tag can then be used to look up the window handle the
renderer is supposed to render into.
OwrVideoRenderer: Make use of the tag=handle window registry

The window handle is looked up from the window registry. If none is
found, the renderer will create its own window, which preserves the
previous default behaviour. Otherwise the window handle will be set on
glimagesink and glimagesink will render into that widget.