Issue #4102. Solving sorting issue on the back-end of the menu manager.

This pull request close the issue #4102 .

This issue was affecting the ordering property. This property contained an array with the next structure:

    [
        menu_id => [
             menuitem_id1
             , menuitem_id2
             , ...
        ]
    ]

But with the code I've added to it, the structure will be as follows:

    [
        menu_id => [
            lft_property1 => menuitem_id1
            , lft_property2 => menuitem_id2
            , ...
        ]
    ]

Adding the lft property, we would know the position related to the other menu items.