Submenu

Create a new kind, onyx.Submenu. Submenus are meant to live inside of onyx.Menus, (as siblings to onyx.MenuItems), and allow for grouping of menu items. This allows for the creation of nested menus (such as the standard Edit menu in webos 2.x)

Implementing the submenu is as transparent as possible, EG:

    {kind:"onyx.Menu", components:[
        {content:"One"},
        {kind:"onyx.Submenu", content:"Two...", components:[
            {content:"A"},
            {content:"B"},
            {content:"C"},
        {content:"Three"}
    ]},

Submenus use @onyx-gradient-inverted as their background, and look like this:
![example](https://f.cloud.github.com/assets/779770/326734/bd1f1d28-9b45-11e2-9bbe-5b5f05b8b298.png)

Enyo-DCO-1.1-Signed-off-by: Chris Mondok <chris.mondok@gmail.com>