Properly convert @longdesc and @aria-describedat to EPUB 3

-  all resource manipulation is now performed in `html-to-epub3.content.xpl`
- resources URIs are sanitized (safe URIs, remove non-existing)
- if image descriptions are found, try to convert DIAGRAM to HTML
- `@longdesc` and `@aria-describedat` are converted and/or doubled with
  `@aria-describedby` pointing to CSS-hidden asides with `iframes`.
- more tests