Change the control visibility by using css class

Instead of changing the button opacity in the control, only add/remove
a css class (ol-hidden). This let the developer easily change the
behavior by updating the css.

src/ol/css.js

@@ -1,6 +1,15 @@
 goog.provide('ol.css');
 
 
+/**
+ * The CSS class for hidden feature.
+ *
+ * @const
+ * @type {string}
+ */
+ol.css.CLASS_HIDDEN = 'ol-hidden';
+
+
 /**
  * The CSS class that we'll give the DOM elements to have them unselectable.
  *