DocBook writer: output identifiers of figure blocks

[Flupp]

@reviewer: I am not sure if the id needs to be prefixed with writerIdentifierPrefix opts as done in line 190 (this is the only place where the prefix is added AFAICT).

[Flupp]

Note: In contrast to the DocBook writer, the DocBook reader already supports the identifier.

Behavior before the change:

$ pandoc -f Markdown -t docbook <<<'![caption text](file.svg){#testid}' | pandoc -f docbook -t markdown 
![caption text](file.svg "fig:")

Behavior after the change:

$ pandoc -f Markdown -t docbook <<<'![caption text](file.svg){#testid}' | pandoc -f docbook -t markdown
![caption text](file.svg "fig:"){#testid}

[jgm]

Docs for --id-prefix say "Specify a prefix to be added to all identifiers and internal links in HTML and DocBook output," so yes, I'd say it should be added in this case too.

[Flupp]

I’ve added the prefix. However, now I seriously wonder if the prefix is maybe also missing at other places in the DocBook writer. The following

:::{#bar}
bar div
:::

:::{#sec .section}
sec div
:::

[foo link](#foo)

[span]{#spanid}

![caption text](file.svg){#testid}

translates via pandoc --id-prefix=prefix- -f Markdown -t docbook (including this change) to

<para xml:id="bar">
  bar div
</para>
<para xml:id="sec">
  sec div
</para>
<para>
  <link linkend="prefix-foo">foo link</link>
</para>
<para>
  <anchor xml:id="spanid" />span
</para>
<figure xml:id="prefix-testid">
  <title>caption text</title>
  <mediaobject>
    <imageobject>
      <imagedata fileref="file.svg" />
    </imageobject>
    <textobject><phrase>caption text</phrase></textobject>
  </mediaobject>
</figure>

Note that prefix- is not added to every id.

[jgm]

Hi, I'm sorry I let this languish so long.

You're right that the prefix isn't added everywhere it should be. Perhaps that could be addressed in a separate PR. Or, feel free to address it here.

I'll add one substantive comment to your code above.

[jgm]

It would also be great to have a test. Simplest way is a "command test" called test/command/8608.md. See the other files in that directory for examples.