Support template mimetypes

Unless I forgot something the only difference between template versions and normal documents is the mimetype.

Fixes #744.
Fixes #746. (still not sure if this is the best API)
Meh. I'm clearly struggling to understand what benefit these changelogs should bring to which audience.

I can't comprehend where one finds such a user who understands references to odf.OdfContainer, but doesn't directly read the commit logs themselves. This seems like an odd mix of low-level detail and high-level feature.

Another example of release notes: https://www.winehq.org/announce/1.7.23

Oh well. I have no further opinions to express on this subject, as I don't feel any ability to guess who reads this.