Enable fancytree to support multiple context menus

Every once a while, it is necessary to support different context menus for different type of element, i.e. folder node is created with folder specific menu items, and non-folder node is appended with non-folder specific menu items.

By this way, the only change during initialization will be declare an array of menu as opposed to single menu.
$('#tree-identifier').fancytree(
         ......
         menu: [{
             selector: ".folder-node",
             ......       
         },
         {
             selector: ".item-node",
             ......
         }],
        ......
};