Fix breathing sign Y-offset when using line-positions.

Fix breathing sign Y-offset when using line-positions.

* input/regression/breathing-sign-custom-staff.ly

  new regtest

* lily/breathing-sign.cc (offset_callback):

  use Staff_symbol::line_span () to calculate offset instead of
  Staff_symbol_referencer::line_count (): the latter is only useful for
  symmetrical staves

  tidy code; remove obsolete TODO

[Neil Puttock, 2011-06-15 21:42:09]

This fixes issue #1696, ensuring breathing signs are always positioned on
extremal staff lines.

[Neil Puttock, 2011-06-18 22:56:19]

Pushed: c1a48137f98086b46a91cf5b741c0b744409904b