OpenDocument writer: Implement figure numbering in captions

[pyssling]

Figure captions are now numbered 1, 2, 3, ... The format in
the caption is "Figure 1: " and so on.

This is necessary in order for libreoffice to generate an
Illustration Index (Table of Figures) for included figures.

[jgm]

Instead of hard-coding the word "Figure," you can use the new translations.
See translateTerm and setTranslations in Text.Pandoc.Class.

[pyssling]

I can look at doing it, but as far as I can see no translations have been used this far in any writers, so I really don't know how the API is intended to be used. Would you consider merging this and then adding translations? I plan to do the same numbered captions for tables and then I could use your example for that.

[jgm]

Translations are used in the LaTeX reader. Up til now we've avoided using any English words like "Figure" in any of the writers (though LaTeX adds such words automatically), precisely because we want to avoid English-centrism. But with the translation API, we could start doing so. If you want I could merge this and then add the needed code (which is simple).

[pyssling]

If you could I would very much appreciate it.

I haven't coded much Haskell since university in 2001 and let's say that I've gone through a steep learning curve the last few weeks.

[mb21]

@pyssling see 58ae017 and feel free to ask questions :-) (btw. I found http://learnyouahaskell.com really helpful)