Enable localized figures in ebooks and pdf

Image files living in <lang>/figures and <lang>/figures-dia can be used for pdf and ebook generation.

If they are named `<chapter>.<image_number>.png`, they will override the
corresponding default (english) files. Moreover, for pdf generation, pdf
images can be provided as `<chapter>.<image_number>.pdf`.

Files in <lang>/figures-dia are converted on the fly to the appropriate format for
ebook and pdf.