Enable localized figures in ebooks and pdf

Image files living in <lang>/figures 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`.
Add conversion of dia source images for ebooks and pdfs

The conversion is done on the fly to generated best matched formats of
images for each transformation. The conversion requires obviously `dia` to
be installed. For PDF, additionally, `epstopdf` must be available.