When re-encoding, allow transcoding to add playablefiles rather than replacing them

Say you've already got some playablefiles for a video, and you'd like to add some more. Since transcoding is an expensive operation (computationally, if you're doing it yourself, or $$$ if done through a 3rd party) it makes sense to have the option to either replace all existing playablefiles with the new batch of transcoded files, or add the transcoded files to the existing playablefiles.

My use case is actually for a migration - see #1835478: Migrating to the video field with the migrate module. I have 3 video transcoding presets on my new site: small, medium, and HD. I already have existing transcoded files for small and HD (which I'm planning to import as playablefiles linked to the video field) and then just need to transcode using the medium preset, and add that playablefile to the existing playablefiles.

I see this being accomplished in one of two ways:

* an additional checkbox on the video field form, reading, "Replace existing transcoded files with new transcoded files," or
* a Views Bulk Operation which can queue transcoding for a bunch of nodes and then magically add the results to the existing playablefiles.

Thoughts?