Make Atmos content work more like other content. Now its MXFs

Make Atmos content work more like other content.  Now its MXFs
are re-written, meaning that they can be encrypted.

This commit is mostly just for the backend.  We also need a fair
few checks/restrictions in the UI:

- any present atmos content dictates the project frame rate
- no mixed edit rates of atmos content
- probably some other things I haven't thought of