i18n of program and slideshow

This commit includes:

* i18n of slideshow in po4a system (with Debian-recommended
  paths),
* a .gitignore,
* somewhat simplified slideshow.py and related code,
* somewhat improved and updated slideshow text, partially joined
  with ubiquity's mint-slideshow texts,
* only some four "fuzzy" translations per language (kind to
  your translators),
* complete 'sl' translation as the contributor speaks it perfectly,
* removed (any reason for) en_GB and en_AU -- lots of empirical
  evidence shows only two languages require additional locale
  specified: pt and zh,
* Makefiles for building/updating translations (target 'update'
  for updating program labels while slideshow is auto-updated on each
  build, just keep them *.po files in sync).

**This commit does depend on debian bump commit.**