Make the file entity a classed object

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Sub-issue of #49. Currently Backdrop uses "real" objects only for Comments, since Backdrop was forked just after https://drupal.org/node/1184944, which added the initial conversion of entities into real objects with CRUD support.

Files entities are currently owned by System module. After completing this conversion, it's likely we will follow D8's suit here and move the hook_entity_info() for files into File module after this conversion.

Drupal.org issue: https://drupal.org/node/1361226

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I've started a port of the File entity class at https://github.com/quicksketch/backdrop/compare/152;file_classes?expand=1.

This includes the primary patch at https://drupal.org/node/1361226 as well as the prerequisite from https://drupal.org/node/1401558. We'll see how Travis reports on the tests and then make an official PR.

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Tests are now passing on this PR: backdrop/backdrop#132.

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Let's merge this in so we can move forward with cleaning up the APIs associated with all entity types in #164.

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backdrop/backdrop#132 has been merged in.