Improve image descriptions in zedai-to-html

- support for the basic inline DIAGRAM descriptions produced by
  `dtbook-to-zedai`, i.e. support for DTBook prodnotes
- add `@aria-describedby`  where it makes sense
- no longer discard ZedAI descriptions when another higher-prio
  description was used as alternative text.
- more tests
Improve DIAGRAM to HTML conversion

- fix bugs in the original transformation (from DIAGRAM Center)
- port to XSLT 2.0
- import `zedai-to-html` for ZedAI core elements
Merge pull request #42 from daisy/feature/image-descriptions

Improve conversion of ZedAI and DIAGRAM image descriptions