talkatu/talkatu

Replace the insert-image action with attach-file, add it to html and markdown buffer, and display dialogs up to the point where we can right now.
Talkatu is versioned in [mercurial](https://www.mercurial-scm.org/) and is
available on [bitbucket](https://bitbucket.org/pidgin/talkatu).
All contributions are provided via pull requests to the bitbucket repository.