Refs #485, #504 - Allow adding nested FileUpload files

With a YAML field of `defaultOptions[images]` the new `getColumn()` method returns `$this->model->defaultOptions->images()` as expected. Works infinitely deep.

* This PR is backwards compatible.
* PR #578 is required for adding to work correctly.
* PR #580 is required for removing to work correctly.
* This PR only allows fixes the FileUpload formwidget. Saving forms with nested FileUpload fields still fail due to SQL error *SQLSTATE[42S22]: Column not found: 1054 Unknown column 'defaultOptions[images]' in 'field list'* which is a bug in a different part of October.

[Demonstration repo](https://github.com/Flynsarmy/oc-badfileupload-plugin)