Allowing the CP field form driver to show the title column checkbox.

This is off by default, but can be set by passing allow_title_column_set = true in the $extra array.

Addresses #2483 and #2427
Allowing setting the title column for page fields.

This allows setting a title column for relating.

Addresses #2483 and #2427
Changing my mind on this.

We should give site builders freedom to relate a page stream (but not a page itself).

This just needs to be made clear in the docs.

Closes #2483 and #2427