[Xfce-i18n] New doc translation handling in terminal

[Nick Schermer]

Hi,

I've created a branch for terminal to handle the doc translations a
bit different. Instead of manually adding the translated xml files
from the submitted (using transifex) po files, it now build translated
html pages (using the --enable-gen-doc configure option, requires
xml2po and xsltproc). The dist tarball contains the translated html
files, so distros don't need to do this and can install the docs
directly. Translations that don't have "translated" images also
symlink to the C images, using less disk space and making the tarball
a lot smaller.

So, would be nice if people can look at the code and this can move to
master in a couple of days.

Cheers,
Nick