Improve opacity slider (#215)

- adopt horizontal styles
- common font size for layers tab
- adopt to layer switcher layout with slider as child of label element
- separate slider wrapper from control, as overlay slider is none
- migrate localStorage legacy key

js/control/LayersTab.js

@@ -66,10 +66,10 @@ BR.LayersTab = BR.ControlLayers.extend({
                     self._layers[i].layer.setOpacity(opacity);
                 }
             }
-        }).onAdd(map);
+        });
         L.DomUtil.get(
             'leaflet-control-layers-overlays-opacity-slider'
-        ).appendChild(overlayOpacitySlider);
+        ).appendChild(overlayOpacitySlider.getElement());
     },
 
     initButtons: function() {