handle multiple video output streams

After enabling H.239, Ekiga will need to handle multiple remote video
streams. Currently Ekiga only shows one remote video stream.

This patch sets the basics for multiple remote video streams: each remote
video stream is named uniquely and according to it the video output would be
able to manage the stream.

For now, the patch only shows the first remote video stream to reach the
endpoint, the subsequent is just ignored.

Closes bug #682387.