New instrument name positioning in scheme.

New instrument name positioning in scheme.

* split ly:system-start-text::print into three scheme callbacks in
  output-lib.scm (X-/Y-offset calculations and print function)

* compensate for system start delimiter extents so all instrument names
  in a system are aligned correctly based on self-alignment-X

* use 'when from left-bound instead of get_break_index () to determine
  which text to print; this fixes incorrect indentation and text for
  shortInstrumentName overrides (Issue #452)

* remove acknowledger for system-start-text from
  system-start-delimiter-engraver.cc

* remove system-start-text.cc

* add system-start-text-interface to define-grob-interfaces.scm

* add library functions used by new callbacks to lily-library.scm
  and output-lib.scm

* update snippets broken by changes

* add news entry and convert rule