Moved ``plone.i18n`` dependency to a ``plone`` extra.

[mauritsvanrees]

This is only used for getting language names in the list command. We now fall back to using the language name that is in the po files.

Fixes issue #44.

I have simplified the buildout.cfg with only the few pins that we really need.

When used in plone.app.locales with the original code, the result of i18ndude list -p plone looks like this:

Messages: 2771

Tier 1:

100% - English (en)
 92% - French (fr)
 92% - Italian (it)
 84% - German (de)
 79% - Spanish (es)
 98% - Dutch (nl)
 77% - Chinese (China) (zh-cn)
 95% - Chinese (Taiwan) (zh-tw)
 85% - Japanese (ja)
 10% - Korean (ko)
 90% - Portuguese (Brazil) (pt-br)
 75% - Russian (ru)
 56% - Polish (pl)
 51% - Turkish (tr)
 13% - Thai (th)
 38% - Arabic (ar)

Tier 2:

 62% - Swedish (sv)
 57% - Finnish (fi)
 84% - Danish (da)
 48% - Portuguese (pt)
 59% - Romanian (ro)
 36% - Hungarian (hu)
 40% - Hebrew (he)
 41% - Indonesian (id)
 68% - Czech (cs)
 28% - Greek (el)
 52% - Norwegian (no)
 46% - Vietnamese (vi)
 46% - Bulgarian (bg)
  9% - Croatian (hr)
 37% - Lithuanian (lt)
 53% - Slovak (sk)
  0% - Tagalog (tl)
 79% - Slovenian (sl)
 22% - Serbian (sr)
 62% - Catalan (ca)
 29% - Latvian (lv)
 83% - Ukrainian (uk)
  7% - Hindi (hi)

Tier 3:

 54% - Afrikaans (af)
  3% - Albanian (sq)
  8% - Armenian (hy)
100% - Basque (eu)
  8% - Bengali (bn)
 18% - Burmese (my)
 16% - Chinese (Hongkong) (zh-hk)
 52% - Esperanto (eo)
 17% - Estonian (et)
 17% - Furlan (fu)
 39% - Galician (gl)
 15% - Georgian (ka)
 15% - Kannada (kn)
 22% - Macedonian (mk)
  8% - Nynorsk (nn)
 18% - Persian (fa)
  8% - Tamil (ta)
  8% - Telugu (te)
 18% - Welsh (cy)

The new result without plone.i18n looks like this:

Messages: 2771

Tier 1:

100% - English (en)
 92% - French (fr)
 92% - Italiano (it)
 84% - Deutsch (de)
 79% - Español (es)
 98% - Nederlands (nl)
 77% - Chinese Simplified (zh-cn)
 95% - Traditional Chinese (zh-tw)
 85% - Japanese (ja)
 10% - 한국어 (ko)
 90% - Português do Brasil (pt-br)
 75% - Russian (ru)
 56% - Polish (pl)
 51% - Türkçe (tr)
 13% - Thai (th)
 38% - Arabic (ar)

Tier 2:

 62% - English (sv)
 57% - Suomeksi (fi)
 84% - Danish (da)
 48% - Português (pt)
 59% - Romanian (ro)
 36% - Magyar (hu)
 40% - עברית (he)
 41% - Bahasa Indonesia (id)
 68% - Czech (cs)
 28% - Ελληνικά (el)
 52% - Norsk (no)
 46% - Vietnamese (vi)
 46% - Bulgarian (bg)
  9% - Croatian (hr)
 37% - Lithuanian (lt)
 53% - Slovenčina (sk)
  0% - tl (tl)
 79% - Slovenian (sl)
 22% - Serbian (sr)
 62% - Catalan (ca)
 29% - Latviešu (lv)
 83% - Українська (uk)
  7% - Hindi (hi)

Tier 3:

 54% - Afrikaans (af)
  3% - Albanian (sq)
  8% - Armenian (hy)
100% - Basque (eu)
  8% - Bengali (bn)
 18% - Burmese (my)
 16% - Chinese(HongKong) (zh-hk)
 52% - Esperanto (eo)
 17% - Estonian (et)
 17% - Furlan (fu)
 39% - Galego (gl)
 15% - Georgian (ka)
 15% - Kannada (kn)
 22% - Macedonian (mk)
  8% - Nynorsk (nn)
 18% - Persian (fa)
  8% - Telugu (te)
 18% - Welsh (cy)
  8% - tamil (ta)

So this is not very different. When done in a client project with only Dutch (nl) translations, the new result is:

Messages: 24

Tier 1:

100% - English (en)
  0% - fr (fr)
  0% - it (it)
  0% - de (de)
  0% - es (es)
100% - Nederlands (nl)
  0% - zh-cn (zh-cn)
  0% - zh-tw (zh-tw)
  0% - ja (ja)
  0% - ko (ko)
  0% - pt-br (pt-br)
  0% - ru (ru)
  0% - pl (pl)
  0% - tr (tr)
  0% - th (th)
  0% - ar (ar)

Tier 2:

  0% - sv (sv)
  0% - fi (fi)
  0% - da (da)
  0% - pt (pt)
  0% - ro (ro)
  0% - hu (hu)
  0% - he (he)
  0% - id (id)
  0% - cs (cs)
  0% - el (el)
  0% - no (no)
  0% - vi (vi)
  0% - bg (bg)
  0% - hr (hr)
  0% - lt (lt)
  0% - sk (sk)
  0% - tl (tl)
  0% - sl (sl)
  0% - sr (sr)
  0% - ca (ca)
  0% - lv (lv)
  0% - uk (uk)
  0% - hi (hi)

Which I would say is fine. If anyone actually uses this, and wants the old results, you can pip install i18ndude[plone].