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/index.js

@@ -282,8 +282,9 @@
         }
 
         map.addControl(
-            new BR.OpacitySlider({
+            new BR.OpacitySliderControl({
                 id: 'route',
+                title: i18next.t('map.opacity-slider'),
                 callback: L.bind(routing.setOpacity, routing)
             })
         );