some comments and complaints on the code (issue 5651069)

[janek . lilypond]

Reviewers: milimetr88,

Message:
Hi,

together with Luke (address@hidden) we're preparing a fix for
http://code.google.com/p/lilypond/issues/detail?id=1546

We had a really hard time on Wednesday trying to understand how the
current code works; after 4 hours of reading we have some grasp now.  In
order not to loose this knowledge, we decided to add it as comments -
it'd be good if you confirmed we understood things correctly.
We also added some questions - if you can answer them, it would be
great!

Description:
some comments and complaints on the code

direction.hh: added a #define loop for_UP_and_DOWN()
  for iterating through: {UP, DOWN}
minor changes in note-collision.cc, some comments etc.

Please review this at http://codereview.appspot.com/5651069/

Affected files:
  M flower/include/direction.hh
  M lily/accidental-placement.cc
  M lily/grob.cc
  M lily/include/grob-interface.hh
  M lily/include/note-collision.hh
  M lily/include/note-column.hh
  M lily/include/staff-symbol-referencer.hh
  M lily/note-collision.cc