Add in_selection() to rcube_treelist_widget

So code working on all lists doesn't have to check widget type
to work with selection.

program/js/treelist.js

@@ -96,6 +96,7 @@ function rcube_treelist_widget(node, p)
   this.get_item = get_item;
   this.get_node = get_node;
   this.get_selection = get_selection;
+  this.in_selection = in_selection;
   this.get_next = get_next;
   this.get_prev = get_prev;
   this.get_single_selection = get_selection;
@@ -295,6 +296,15 @@ function rcube_treelist_widget(node, p)
     return selection;
   }
 
+  /**
+   * Check if given id is selected
+   * This is for consistency with rcube_list_widget
+   */
+  function in_selection(id)
+  {
+    return selection == id;
+  }
+
   /**
    * Return the DOM element of the list item with the given ID
    */