Nota 790

Both @stesk and I worked independently on this issue by mistake. We did however to a large extend do complementary work, so no real harm done. We should however probably choose between the two methods for setting epub:type on major document divisions, since they really do the same thing.

This pull-request is the result of merging our efforts.

Comments from @stesk's original pull-request for nota-790:

The modified xhtmlepub.framework contains new Author actions for:

* setting language attributes on the element at the current caret position to one of six values: `da`, `de`, `en`, `fr`, `no`, `se`
* wrapping selected text in a `<span>` element with attributes indicating either of the six languages
* setting `@epub:type` on major content divisions
  * if the book is edited in a one-file format, the attribute will be applied to the topmost `<section>` element below the `<body>` element
  * otherwise, if an individual content document is edited, the attribute will be set on the `<body>` element
  * in either case, the required secondary value (`frontmatter`, `bodymatter` or `backmatter`) is retained or changed in accordance with the provided guidelines
* applying class values to subdivisions of the cover: `frontcover`, `rearcover`, `leftflap` and `rightflap`

The actions have been added to the contextual menu in the Author view.