[www-client/firefox-l10n-*] introduce scripts and eclass

Files to manage separate language packs for Firefox.

- firefox-l10n.eclass
- ebuilds-new-version-Firefox-l10n.sh - install ebuild for the new
  version
- install-Firefox-l10n.sh - install the packages on system

--

The most important question to be asked is: do we want them?
pros:
- A bit smaller disk usage: language files take (using du with no special options) 77M (Entropy package; /usr/lib/firefox/browser/extensions/langpack-*; that's hopefully all), and 157M if all languages are enabled.
- Can provide Firefox language packs for all languages easily and fast, and that doesn't depend on LINGUAS.

neutral:
- Possibly no noticeable gain in startup time (Firefox provided by Entropy seems to start fast, so I didn't measure it further).
- No big maintenance cost: scripts are quick and easy to use. Additional maintenance cost will occur if mozlinguas.eclass or mozextension.eclass (or whatever else, as normal) change so that it needs adjusting. However, the two eclasses don't receive much changes.

cons:
- In this setup, no patches can be applied on the language files (the need is unlikely, though; I asked for confirmation on IRC, but no answer yet).

note:
The script ebuilds-new-version-Firefox-l10n.sh is interactive. It cannot be (easily and robustly) made automated because of the need to extract list of languages (from a Firefox ebuild). There is a note in Firefox ebuilds:
`This list can be updated with scripts/get_langs.sh from the mozilla overlay`
so we may consider trying that if deemed useful. However, there is always a possibility that the resulting list will be different from what is in the ebuild (possibly not a big problem).