Add <ResourceMenuItems> to facilitate custom menus

[fzaninotto]

The <Menu.ResourceItems> component displays one menu item for each resource, using the resource label and icon defined in the <Resource> components, and in the order in which they were declared in <Admin>.

import { Menu } from 'react-admin';

export const MyMenu = () => (
    <Menu>
        ...
        <Menu.ResourceItems />
        ...
    </Menu>
);

So with the following <Admin> component:

const App = () => (
    <Admin dataProvider={dataProvider} layout={MyLayout} dashboard={MyDashboard}>
        <Resource name="posts" list={PostList} icon={BookIcon} />
        <Resource name="comments" list={CommentList} icon={ChatBubbleIcon} />
        <Resource name="tags" list={TagList} />
        <Resource name="users" list={UserList} icon={PeopleIcon} />
    </Admin>
);

<Menu.ResourceItems> renders the following menu items:

<Menu.ResourceItem name="posts" />
<Menu.ResourceItem name="comments" />
<Menu.ResourceItem name="tags" />
<Menu.ResourceItem name="users" />