Up until now, only main menu items have supported the 'order' parameter
to set the sort order. This adds the same support to side menus, which
is especially useful when one plugin extends the side menu of another.
There is one downside: any existing plugins that use the side menu will
inevitably end up with their menu items appearing out of order, until
the author adds the 'order' parameter to their sideMenu array.
e6342dbd8a