p-d-e doesn't support building translated help

Currently p-d-e does not support building localized application help. There are several applications[1] which use p-d-e and have user help, but the help is currently not translatable using this build system.

Features needed to be supported:
 - building a help pot file to be translated.
 - merging po translations into xml documentation, using itstool for mallard and xml2po for docbook.
 - validate merged translations with xmllint. Help usually contains a lot of xml, which translators tend to mess up, reporting such cases would be nice.
 - (un)installing translated xml files and translated figures under standard help paths - build_help already does this for the C language code
 - optionally: merging existing po translations under $help_dir/$ll into the new pot file. Launchpad can do this for us, and Damned-lies does this for upstream Gnome too, but there may be folks out there not using these tools.

For autotools, this script does the magic:
http://git.gnome.org/browse/yelp-tools/tree/tools/yelp.m4

[1]: for example software-center, language-selector, usb-creator