Reorganize language files and add a new \language command. (issue2699041)

[v . villenave]

Reviewers: ,

Message:
Greetings everybody.

Third time's a charm: here's a new attempt to get the \language "foobar"
syntax merged. Cons: this patch does move a lot of files around. Pros:
this is a lot less hackish that my two previous attempts.

Please have a look and feel free to comment; if this patch is approved,
then I'll upload a second patch set to handle Documentation changes!

Thanks to Neil and Carl for their comments and suggestions. Hopefully
this time I got it right... :-)

Description:
Reorganize language files and add a new \language command.

This commit adds a new \language command that may be used
to select note names language in lieu of the more complicated
\include "foobar.ly" syntax.

All language files have been emptied and the pitchnames alists
have been gathered in a new language-init.ly file.

The old syntax is still supported; to avoid code duplication,
language files just use the new \language command internally.

arabic.ly isn't affected by this change and must still be
manually included.  This specificity will have to be taken
into account when documenting the new syntax.

Please review this at http://codereview.appspot.com/2699041/

Affected files:
  M Documentation/changes.tely
  M ly/arabic.ly
  M ly/catalan.ly
  M ly/declarations-init.ly
  M ly/deutsch.ly
  M ly/english.ly
  M ly/espanol.ly
  M ly/italiano.ly
  M ly/music-functions-init.ly
  M ly/nederlands.ly
  M ly/norsk.ly
  M ly/portugues.ly
  M ly/suomi.ly
  M ly/svenska.ly
  M ly/vlaams.ly