Add support for video thumbnails

[robUx4]

In adaptive streaming (DASH-IF) there is a way to provide a picture with a mosaic of thumbnails for a video stream.

This is provided in this form:

<AdaptationSet id="3" mimeType="image/jpeg" contentType="image">
<SegmentTemplate media="$RepresentationID$/tile_$Number$.jpg" duration="634.566" startNumber="1"/>
<Representation bandwidth="12000" id="thumbnails_102x58" width="1024" height="1152">
<EssentialProperty schemeIdUri="http://dashif.org/thumbnail_tile" value="10x20"/>
</Representation>
<Representation bandwidth="24000" id="thumbnails_256x144" width="2048" height="1152">
<EssentialProperty schemeIdUri="http://dashif.org/thumbnail_tile" value="8x8"/>
</Representation>
</AdaptationSet>

We should be able to store similar variants in Matroska.

One tricky part is when the file is edited/cut the thumbnails become invalid. Or there should be a way to keep it and define the valid parts.

[robUx4]

VLC will add support if we define a way to do this: https://code.videolan.org/videolan/vlc/-/issues/29393