Rename lessc->embeddedImages to lessc->embeddedFiles

Rename lessc->embeddedImages to lessc->embeddedFiles

'embeddedImages' is a custom property we set on lessc compiler instances to
track the set of files that were embedded in the generated source as data URIs.
This is so done so we know to regenerate the CSS if the source file for an
embedded asset changes. (If my pull request[1] is merged, we won't have to use
a custom property at all.)

I realized that the property name should be 'embeddedFiles' rather than
'embeddedImages', because it's not just images that can be usefully embedded in
CSS, but fonts as well.

  [1]: https://github.com/leafo/lessphp/pull/472

Change-Id: Ief3afaa23e4532f4a536e0dfef943d4fa20db80d