* Documentation/topdocs/NEWS.tely (Top): document new feature.

* Documentation/topdocs/NEWS.tely (Top): document new feature.

* Documentation/user/basic-notation.itely (Staff symbol): document
start/stop staff. Reference to ossia.ly

* input/test/ossia.ly: new example using stop and startStaff.

* lily/timing-engraver.cc: remove Timing_engraver::initialize(),
in other words, don't create "|" barline at start of the score.
(start_translation_timestep): don't set whichBar for start of score.

* input/regression/staff-halfway.ly: use new functionality.

* lily/staff-symbol-engraver.cc (try_music): take StaffSpanEvents,
start and stop staff based on events.

* scm/define-music-types.scm (music-descriptions): add StaffSpanEvent

* ly/declarations-init.ly (startStaff, stopStaff): new identifiers.