One last change for #1294.

This should make the initial values of the edit title/description
dialog correct.  The new system is:

- For title:
  - The we take it from the language data
  - if that's not set, we take it from the video's title.
- For description and other metadata:
  - We take it from the language data
  - If that's not set, and we're editing the primary audio language,
    then we take it from the video's description/metadata.