line_count related patches in a single commit for review (issue 6419064)

[benko . pal]

Reviewers: Keith, dak, marc,

Message:
when splitting the previous patch into smaller commits I added new
regtests and found further errors with bar-line.
to fix those there are some changes from the previous version:
- calc_bar_extent now not only shrinks the bar line, but can expand it
for narrow (e.g. single-line) staves
- compound_barline is restructured

Description:
line_count related patches in a single commit for review

in decreasing order of importance:
- make slurs symmetric to staff
- improve time signature and bar line (in particular dots of a repeat
sign)
  positioning under non-standard circumstances
- enhance regtests
- etc.

Please review this at http://codereview.appspot.com/6419064/

Affected files:
  M input/regression/ledger-lines-varying-staves.ly
  M input/regression/non-centered-bar-lines.ly
  A input/regression/repeat-sign-global-size-10.ly
  A input/regression/repeat-sign-global-size-30.ly
  A input/regression/repeat-sign-layout-size.ly
  A input/regression/repeat-sign.ly
  M input/regression/staff-ledger-positions.ly
  M input/regression/staff-line-positions.ly
  M lily/bar-line.cc
  M lily/beam.cc
  M lily/breathing-sign.cc
  M lily/custos.cc
  M lily/rest-collision.cc
  M lily/rest.cc
  M lily/slur-scoring.cc
  M lily/time-signature.cc
  M lily/vaticana-ligature.cc