Fixed requesting of legend graphics each time the capabilities are requested

[lgoltz]

Before this PR the configured legend is requested each time the capabilities are requested to determine the size of the legend:

<l:StyleRef>		
  <l:StyleStoreId>styles</l:StyleStoreId>		
    <l:Style>
      <l:StyleName>test</l:StyleName>
      <l:LayerNameRef>test_layer</l:LayerNameRef>
      <l:StyleNameRef>test</l:StyleNameRef>
      <l:LegendGraphic outputGetLegendGraphicUrl="false">https://deegree.org/legends/legend.png</l:LegendGraphic>
    </l:Style>
</l:StyleRef>

This PRs fixes this by storing the legend size in the styles instance.

[dstenger]

A functional test was successful.
main-Branch: When using the snippet documented in the description, the legend is requested when requesting the capabilities. This applies to all additional requests.
Branch of this PR: When using the snippet documented in the description, the legend is only requested when requesting the capabilities for the first time. Afterwards, the legend is not requested anymore.

[stephanr]

As a small remark, this may interfere in a setup where the legend graphic on disk or remote changes (in size) over time.