[FIX] web_editor: review _setActive system for select-class

- Handle the case where one select-class is a subset of another
  select-class of the same group e.g.

  <... data-select-class="hello"/>
  <... data-select-class="hello world"/>

- Handle empty values <... data-select-class=""/>

Note: both cases worked but were not displaying the active checkbox
correctly in the editor UI.

addons/web_editor/static/src/js/editor/snippets.options.js

@@ -275,15 +275,36 @@ var SnippetOption = Widget.extend({
      */
     _setActive: function () {
         var self = this;
-        this.$el.find('[data-toggle-class], [data-select-class]')
-            .addBack('[data-toggle-class], [data-select-class]')
+        this.$el.find('[data-toggle-class]')
+            .addBack('[data-toggle-class]')
             .removeClass('active')
             .filter(function () {
-                var $elem = $(this);
-                var className = $elem.data('toggleClass') || $elem.data('selectClass');
-                return self.$target.hasClass(className);
+                var className = $(this).data('toggleClass');
+                return !className || self.$target.hasClass(className);
             })
             .addClass('active');
+
+        _processSelectClassElements(this.$el);
+        _.each(this.$el.find('.dropdown-menu'), function (group) {
+            _processSelectClassElements($(group).children());
+        });
+
+        function _processSelectClassElements($elements) {
+            var maxNbClasses = -1;
+            $elements.filter('[data-select-class]')
+                .removeClass('active')
+                .filter(function () {
+                    var className = $(this).data('selectClass');
+                    var nbClasses = className ? className.split(' ').length : 0;
+                    if (nbClasses >= maxNbClasses && (!className || self.$target.hasClass(className))) {
+                        maxNbClasses = nbClasses;
+                        return true;
+                    }
+                    return false;
+                })
+                .last()
+                .addClass('active');
+        }
     },
 
     //--------------------------------------------------------------------------