add a bare list of tracks to group them together

Author: robUx4

ebml_matroska.xml

@@ -1285,6 +1285,15 @@ A Matroska Player **MAY** support encryption.</documentation>
       <enum value="2" label="MD5"/>
     </restriction>
   </element>
+  <element name="TrackSets" path="\Segment\Tracks\TrackSets" id="0x747B" type="master" maxOccurs="1">
+    <documentation lang="en" purpose="definition">The list of TrackSet.</documentation>
+  </element>
+  <element name="TrackSet" path="\Segment\Tracks\TrackSets\TrackSet" id="0x7428" type="master" minOccurs="1">
+    <documentation lang="en" purpose="definition">Describes a list of tracks that **MUST** be presented together when a `Matroska Player` supports the TrackSet feature.</documentation>
+  </element>
+  <element name="TrackSetUID" path="\Segment\Tracks\TrackSets\TrackSet\TrackUID" id="0x7423" type="uinteger" minver="5" range="not 0" minOccurs="1">
+    <documentation lang="en" purpose="definition">The `TrackUID` of a track belonging to the `TrackSet`.</documentation>
+  </element>
   <element name="Cues" path="\Segment\Cues" id="0x1C53BB6B" type="master" maxOccurs="1">
     <documentation lang="en" purpose="definition">A Top-Level Element to speed seeking access.
 All entries are local to the Segment.</documentation>