Reorganize language files and add a new \language command.

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.