Support dot syntax in form field names

[sminnee]

It would be great if saveInto() and loadDataFrom() supported the dot syntax for traversing has-one relations. If you had Book has_one Author, it would let you set up forms such as this:

new FieldList(
  new TextField('Title', 'Book title'),
  new TextField('Author.FirstName', 'Author firstname'),
  new TextField('Author.Surname', 'Author surname'),
);

[sminnee]

#9162 relates to this, in that they're both very helpful for building better ModelAdmins around many-many-through datasets. The boilerplate listed in that ticket is the workaround that you need because this feature isn't available.