Add ability to set <SimpleShowLayout direction>

[fzaninotto]

Problem

Writing a custom show layout by hand is cumbersome, because we have to decorate each field with <Labeled>.

Solution

Allow users to nest <SimpleShowLayout> of various direction.

export const BookShow = () => (
    <Show>
        <SimpleShowLayout>
            <TextField source="title" />
            <TextField source="summary" />
            <SimpleShowLayout
                direction="row"
                sx={{
                    padding: 0,
                    '& .ra-field-id': { width: 50 },
                    '& .ra-field-author': { width: 150 },
                }}
            >
                <TextField source="id" />
                <TextField source="author" />
                <NumberField source="year" />
            </SimpleShowLayout>
        </SimpleShowLayout>
    </Show>
);