Improve conversion of ZedAI and DIAGRAM image descriptions

- fix bugs in the original `diagram-to-html.xsl` transformation (from the DIAGRAM Center)
- support for the basic inline DIAGRAM descriptions produced by
  `dtbook-to-zedai`, i.e. support for DTBook prodnotes in `dtbook-to-epub3`
- add `@aria-describedby`  where it makes sense in `zedai-to-html`
- no longer discard ZedAI descriptions when another higher-prio
  description was used as alternative text.
- more tests