Issue 3299: better alignment with PaperColumn parent (issue 132280043 by address@hidden)

[janek . lilypond]

Reviewers: ,

Message:
Please review.  I hope that the regtest (see output here
https://code.google.com/p/lilypond/issues/detail?id=3299#c9) and commit
message are clear.

best,
Janek

Description:
Issue 3299: better alignment with PaperColumn parent

PaperColumns are the objects that define where a musical moment
"happens" on the staff horizontally.  However, the noteheads aren't
center-aligned on PaperColumns - according to notation rules they
are left-aligned.  Because of that an object centered on a notehead
is not actually centered on the PaperColumn, which results in a
discrepancy between objects attached to NoteHeads and PaperColumns.

This commit fixes that by introducing a placeholder extent which is
used for aligning such grobs.

Please review this at https://codereview.appspot.com/132280043/

Affected files (+162, -49 lines):
  M Documentation/notation/vocal.itely
  A input/regression/alignment-of-grobs-attached-to-paper-column.ly
  D input/regression/unassociated-lyrics-alignment.ly
  M lily/paper-column.cc
  M lily/self-alignment-interface.cc
  M scm/output-lib.scm