Update <SimpleList> to fallback to recordRepresentation when not given primaryText

[fzaninotto]

Problem

In most cases, the primaryText of a SimpleList is the resource's resordRepresentation. If feels repetitive to have to pass an explicit value to this prop.

Solution

Default to recordRepresentation so we can do the following:

Const PostList = () => (
  <List>
    <SimpleList />
  </List>
)

I took the opportunity to include the follow-up to #8925 (deprecating the rowStyle prop in favor of rowSx).