Add ability to pass multiple children to <List> and <Create>

[fzaninotto]

Problem

It's not possible to pass utility components as children of <List> in addition to a <Datagrid>. In particular, I need to pass a <ListLiveUpdate> component to watch for real-time changes in the list, and it needs access to the ListContext.

Solution

Change the accepted type and proptype

const PostList = () => (
  <List>
    <ListLiveUpdate />
    <Datagrid>
      // ...
    </Datagrid>
  </List>
);

Other page components (<Edit> and <Show>) already allow that.