NEWS docs for 2.12

[Graham Percival]

I don't think we need to update the 2.11 docs for NEWS items.  Most 
items have already been added to the docs.  I'm not certain if the below 
items have been added, but IMO none of them are vitally important.

I'd rather not mess around with the 2.11 docs, because most of the 
changes to 2.11 can't be ported to GDP automatically.  These changes 
_will_ make it into GDP.  I know this is a bit lazy of me, but... well, 
I think it's better to spend my effort on GDP instead.

I should have done this stuff _before_ I branched GDP, but I completely 
forgot about the NEWS section.  Sorry.

Cheers,
- Graham

----
A new page breaking function, ly:minimal-breaking, is dedicated to books 
with many pages or a lot of texts.

Page breaking and page turning commands (\pageBreak, \noPageBreak, etc) 
can be used at top-level, between scores and top-level markups.

Nested properties, such as details in Slur, can be reverted as well. The 
syntax for this is

          \revert Slur #'(details closeness-factor)

All \scores in a lilypond-book fragment are now inserted into the 
document. Also, toplevel markups don't result in an entire page.

Tie directions may be set with ^~ and _~

Tablature now supports harmonics and slides,