[UX] Move the Title field label setting to the "Manage fields" form

This is part of #779 and the same as https://www.drupal.org/node/1513054

Contrary to #933, which is about making title a proper Field API field (which is what the Title module does) and which we determined that can only happen in Backdrop 2.x, this issue here is to simply move things around, and sorta "fake" the title being a Field API field (what the Title field UI and Title Display modules seem to be doing).

After implementing this, we could loose the whole "submission settings" tab by moving the "Explanation or submission guidelines" text field out of the tab set and under the content type "Description" text field.