Issue 3728: Allow score and book identifiers at top level, and score identifiers...

Issue 3728: 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.