hls: Add hlswebvttsink element

Closed Seungha Yang requested to merge seungha.yang/gst-plugins-bad:hls-vtt into master
    hls: Add hlswebvttsink element

    Introduce a new hlssink element for producing WebVTT playlist.
    webvttenc: Add support for fragmented webvtt

    Adding a new property "fragment-duration" so that webvttenc element
    can aggregate and/or split text based on configured duration.
    Fragmented WebVTT can be used for HLS or DASH (in WebVTT sidecar form)
Edited by Seungha Yang

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