Add Update to NetworkLinkControl

[cleder]

Follow up to #390, blocks #317

Add missing update functionality to NetworkLinkControl with the Delete, Create and Change objects:

<NetworkLinkControl>      
      <Update>
        <targetHref>http://barcelona.galdos.local/svn1/sqa/ets-kml/main/test/data/ogc-kml/files/ge_.kmz</targetHref>
        <Delete>
          <PhotoOverlay targetId="id_550e8400-e29b-41d4-a716-446655440000" />
        </Delete>
        <Create>
          <Folder targetId="Folder-001">
			<name>Mauris vulputate pulvinar.</name>
          </Folder>
        </Create>
        <Change>
          <TimeSpan targetId="id_f272fc30-185d-11dd-bd0b-0800200c9a66">
            <begin>2008-03-15T00:00:00Z</begin>
            <end>2008-03-29T00:00:00Z</end>
          </TimeSpan>
        </Change>
      </Update>
    </NetworkLinkControl>

Syntax

<Update>
  <targetHref>...</targetHref>    <!-- URL -->
  <Change>...</Change>
  <Create>...</Create>
  <Delete>...</Delete>
</Update>

Description
Specifies an addition, change, or deletion to KML data that has already been loaded using the specified URL. The specifies the .kml or .kmz file whose data (within Google Earth) is to be modified. <Update> is always contained in a NetworkLinkControl. Furthermore, the file containing the NetworkLinkControl must have been loaded by a NetworkLink. See the "Topics in KML" page on Updates for a detailed example of how Update works.

Elements Specific to Update
Can contain any number of <Change>, <Create>, and <Delete> elements, which will be processed in order.

https://developers.google.com/kml/documentation/kmlreference#example-of-change

<Update>
  <targetHref>...</targetHref>    <!-- URL -->
  <Change>...</Change>
  <Create>...</Create>
  <Delete>...</Delete>
</Update>

https://developers.google.com/kml/documentation/kmlreference#example-of-create
https://developers.google.com/kml/documentation/kmlreference#example-of-delete