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