Add support for custom ledger positions, using two new staff-symbol properties (issue 4974075)

[pierstitus]

Reviewers: ,

Message:
Please review my contribution to this wonderful project. Maybe still in
time for 2.16 :)
On the other hand, it changes quite a bit of the way ledger lines are
handled, and I'm not sure what your policy for new features just before
release is.

Piers

Description:
Add support for custom ledger positions, using two new staff-symbol
properties

See http://code.google.com/p/lilypond/issues/detail?id=1292 for the
proposal.

This also adds support for internal ledger lines, and hence resolves
issue http://code.google.com/p/lilypond/issues/detail?id=1193

A regression test for ledger positions is added.

This causes one change in the current regression tests, namely an
internal ledger line is added to staff-line-positions.ly. I don't think
it is a problem to have internal ledger lines by default.

Please review this at http://codereview.appspot.com/4974075/

Affected files:
  A input/regression/staff-ledger-positions.ly
  M lily/include/staff-symbol.hh
  M lily/ledger-line-spanner.cc
  M lily/staff-symbol.cc
  M scm/define-grob-properties.scm