substantial changes in alignment interface - please review

[Janek Warchoł]

Hi folks,

after some work on self-alignment-interface i have reached the point
when a big picture starts to emerge.  I'd really appreciate some
feedback, also from people who are not really into Lily internals (i'd
like to know whether user interfaces produced by me are clear).

Rietveld issue is here: https://codereview.appspot.com/7768043, but i
*strongly* recommend reviewing these changes in a branch, so that you
can see each of 11 commits separately.
You can see them in dev/janek-alignment (commit c204c4a5bc865).
I've also put these changes up on Github
(https://github.com/janek-warchol/lilypond-git/commits/dev/janek-alignment),
because Github allows to post inline comments while still viewing
commits separately - maybe we should review all our code there?

If you need some explanation to get an idea of what this code is doing
at all, look here:
http://lists.gnu.org/archive/html/lilypond-user/2013-03/msg00956.html

What i'm going to do next?  Add support fot alining on different
extents; you can get an idea here:
http://lists.gnu.org/archive/html/lilypond-devel/2012-06/msg00230.html

i'm looking forward to reading your opinions,
Janek