Add ExtendedData and Model to gx:Track

[cleder]

https://developers.google.com/kml/documentation/kmlreference#example-of-track-with-extended-data

    <Schema id="schema">
      <gx:SimpleArrayField name="heartrate" type="int">
        <displayName>Heart Rate</displayName>
      </gx:SimpleArrayField>
      <gx:SimpleArrayField name="cadence" type="int">
        <displayName>Cadence</displayName>
      </gx:SimpleArrayField>
      <gx:SimpleArrayField name="power" type="float">
        <displayName>Power</displayName>
      </gx:SimpleArrayField>
    </Schema>

      <Placemark>
        <name>2010-05-28T01:16:35.000Z</name>
        <styleUrl>#multiTrack</styleUrl>
        <gx:Track>
          <when>2010-05-28T02:02:09Z</when>
          <when>2010-05-28T02:02:35Z</when>
          <when>2010-05-28T02:02:44Z</when>
          <when>2010-05-28T02:02:53Z</when>
          <when>2010-05-28T02:02:54Z</when>
          <when>2010-05-28T02:02:55Z</when>
          <when>2010-05-28T02:02:56Z</when>
          <gx:coord>-122.207881 37.371915 156.000000</gx:coord>
          <gx:coord>-122.205712 37.373288 152.000000</gx:coord>
          <gx:coord>-122.204678 37.373939 147.000000</gx:coord>
          <gx:coord>-122.203572 37.374630 142.199997</gx:coord>
          <gx:coord>-122.203451 37.374706 141.800003</gx:coord>
          <gx:coord>-122.203329 37.374780 141.199997</gx:coord>
          <gx:coord>-122.203207 37.374857 140.199997</gx:coord>
          <ExtendedData>
            <SchemaData schemaUrl="#schema">
              <gx:SimpleArrayData name="cadence">
                <gx:value>86</gx:value>
                <gx:value>103</gx:value>
                <gx:value>108</gx:value>
                <gx:value>113</gx:value>
                <gx:value>113</gx:value>
                <gx:value>113</gx:value>
                <gx:value>113</gx:value>
              </gx:SimpleArrayData>
              <gx:SimpleArrayData name="heartrate">
                <gx:value>181</gx:value>
                <gx:value>177</gx:value>
                <gx:value>175</gx:value>
                <gx:value>173</gx:value>
                <gx:value>173</gx:value>
                <gx:value>173</gx:value>
                <gx:value>173</gx:value>
              </gx:SimpleArrayData>
              <gx:SimpleArrayData name="power">
                <gx:value>327.0</gx:value>
                <gx:value>177.0</gx:value>
                <gx:value>179.0</gx:value>
                <gx:value>162.0</gx:value>
                <gx:value>166.0</gx:value>
                <gx:value>177.0</gx:value>
                <gx:value>183.0</gx:value>
              </gx:SimpleArrayData>
            </SchemaData>
          </ExtendedData>
        </gx:Track>
      </Placemark>

Schema:

<complexType name="TrackType">
    <complexContent>
      <extension base="kml:AbstractGeometryType">
        <sequence>
          <element ref="kml:extrude" minOccurs="0"/>
          <element ref="kml:tessellate" minOccurs="0"/>
          <element ref="kml:altitudeModeGroup" minOccurs="0"/>
          <element ref="kml:when" minOccurs="0" maxOccurs="unbounded"/>
          <element ref="gx:coord" minOccurs="0" maxOccurs="unbounded"/>
          <element ref="gx:angles" minOccurs="0" maxOccurs="unbounded"/>
          <element ref="kml:Model" minOccurs="0"/>
          <element ref="kml:ExtendedData" minOccurs="0"/>
          <element ref="gx:AbstractTrackSimpleExtensionGroup" minOccurs="0"
            maxOccurs="unbounded"/>
        </sequence>
      </extension>
    </complexContent>
  </complexType>

  <element name="SimpleArrayField" type="gx:SimpleArrayFieldType"
    substitutionGroup="kml:SchemaExtension"/>

  <complexType name="SimpleArrayFieldType">
    <sequence>
      <element ref="kml:displayName" minOccurs="0"/>
      <element ref="gx:SimpleArrayFieldExtension" minOccurs="0"
        maxOccurs="unbounded"/>
    </sequence>
    <attribute name="type" type="string"/>
    <attribute name="name" type="string"/>
  </complexType>
  <element name="SimpleArrayFieldExtension" abstract="true"/>

  <element name="SimpleArrayData" type="gx:SimpleArrayDataType"
    substitutionGroup="kml:SchemaDataExtension"/>

  <complexType name="SimpleArrayDataType">
    <complexContent>
      <extension base="kml:AbstractObjectType">
        <sequence>
          <element ref="gx:value" minOccurs="0" maxOccurs="unbounded"/>
          <element ref="gx:SimpleArrayDataExtension" minOccurs="0"
            maxOccurs="unbounded"/>
        </sequence>
        <attribute name="name" type="string"/>
      </extension>
    </complexContent>
  </complexType>