Add <audio> and <video> player backends
These patches enables audio and video playing inside Servo.
It is bit hackish way to enable it, thus the purpose of this pull request is for an early request for comments.
It is tested with the current servo-media GStreamer backend in Linux.
~~The produced layout is not correct, since the elements after the video seems to be stacked behind, and the same with the scrolling bars.~~
~~There is no JavaScript interface yet~~, neither controls.
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- [X] These changes fix#6711
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