Allow score and book identifiers at top level, and score identifiers in book(part)s

Allow score and book identifiers at top level, and score identifiers in book(part)s

Instead of

xxx = \score { ... }
yyy = \book { ... }
\score { \xxx }
\book { \yyy }

you can now just write

xxx = \score { ... }
yyy = \book { ... }
\xxx
\yyy

Book and bookparts at top level are distinguished by virtue of only
books having a \paper block.


Also contains commit:

Regularize score handling (origin and copying)

Scores get their origin set when score_body is called, and when encountered
in \xxx and  constructs, are properly copied.

[lemzwerg, 2013-12-15 00:08:23]

LGTM.