Add the ability to style the layer switcher via behaviors

The attached patch allows you to override the available layer switcher properties (http://dev.openlayers.org/releases/OpenLayers-2.10/doc/apidocs/files/Ope...) through openlayers behaviors, E.g.,

/**
 * Implement hook_openlayers_map_preprocess_alter
 */
function map_openlayers_map_preprocess_alter(&$map = array()) {
  // turn off rounedCorner on the layer switcher
  $map['behaviors']['openlayers_behavior_layerswitcher']['roundedCorner'] = FALSE;
}

I believe this was the module author's intention all along, but "ascending" was the only available property. This adds roundedCorner and roundedCornerColor.