optional configuration parameter 'wmsQueryLayers' to 'wmsLayer'

this makes it possible to have a wmsLayer configuration like this:

    <wmsLayer>
        <name>composite</name>
        <wmsLayers>background,cells</wmsLayers>
        <queryable>true</queryable>
        <wmsQueryLayers>cells</wmsQueryLayers>
    </wmsLayer>
 
on the wms server, the cells layer is queryable, but the background layer is not.