Page breaking fails for multiline embedded score

[Boris Shingarov]

Neil's recent modification to
(define-builtin-markup-command (score ...) ...), in 
scm/define-markup-commands.scm, fixes the bug where only the first system was 
printed, and now we can have multi-line embedded scores, by vertically stacking 
the stencils instead of just selecting the first one. 

While this is a definite improvement over the old buggy behavior, the fact that 
the whole embedded score appears as one monolithic stencil to the markup-list 
word wrapper, leads to a problem with page breaking: namely, the embedded score 
can not be broken across pages. 

Use case 1:
There is half a page of markup, followed by a page worth of embedded score.  
Lilypond will produce a half-empty first page containing markup, and the score 
will follow on the next page. 
What the user expects to see, is the embedded score to be broken across pages. 

Use case 2:
Even worse, a very long embedded score can not be broken across pages at all; 
it is not possible at all to print an embedded score that does not fit on one 
page. 

The fix, I believe, is to not perform the
(stack-stencils Y DOWN baseline-skip ...) application; instead, leave just the 
(map paper-system-stencil ...) so each system becomes its own member of the 
markup list; then let the word wrapper place them vertically.  Because the 
score line-breaking was already done by the Constrained breaker, I hope the 
(less intelligent) word wrapper will not screw the line layout (I don't see 
how, they are all page-wide lines).