* lily/tab-staff-symbol-engraver.cc: derive from Engraver, not

* lily/tab-staff-symbol-engraver.cc: derive from Engraver, not
Staff_symbol_engraver

* ly/engraver-init.ly (AncientRemoveEmptyStaffContext): don't
replace staff_engraver

* lily/staff-symbol-engraver.cc (stop_spanner): announce end of
spanner.

* lily/staff-collecting-engraver.cc
(acknowledge_end_staff_symbol): new function

* lily/volta-engraver.cc (acknowledge_end_staff_symbol): new function.

* lily/staff-collecting-engraver.cc
(acknowledge_end_staff_symbol): new function.

* lily/main.cc: switch on relocation by default.