Configure legend graphis for group layers

[lgoltz]

For group layers a legend graphic should be configurable. The legend graphic can be referenced as local file or remote URL.

Enhancement Themes configuration (proposal):

<?xml version="1.0"?>
<Themes xmlns="http://www.deegree.org/themes/standard" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://www.deegree.org/themes/standard themes.xsd" xmlns:d="http://www.deegree.org/metadata/description">

  <LayerStoreId>example</LayerStoreId>

  <Theme>
    <d:Title>root</d:Title>
    <Theme>
      <Identifier>single</Identifier>
      <d:Title>single</d:Title>
      <Layer layerStore='example'>zipcodes</Layer>
    </Theme>
    <Theme>
      <Identifier>grouplayer</Identifier>
      <d:Title>grouplayer</d:Title>
      <LegendGraphic>grouplayer_legend.png</LegendGraphic>
      <Layer layerStore='example'>layer1</Layer>
      <Layer layerStore='example'>layer2</Layer>
    </Theme>
    <Theme>
      <Identifier>grouplayercomplex</Identifier>
      <d:Title>grouplayercomplex</d:Title>
      <LegendGraphic>https://example.de/grouplayercomplex_legend.png</LegendGraphic>
      <Theme>
        <Identifier>sublayer1</Identifier>
        <d:Title>sublayer1</d:Title>
        <Layer layerStore='example'>sublayer1</Layer>
      </Theme>
      <Theme>
        <Identifier>sublayer2</Identifier>
        <d:Title>sublayer2</d:Title>
        <Layer layerStore='example'>sublayer2</Layer>
      </Theme>
    </Theme>
    
  </Theme>

</Themes>

[tfr42]

The TMC members have seen the config example and consider this extension as reasonable.