* ly/engraver-init.ly (AncientRemoveEmptyStaffContext): remove

* ly/engraver-init.ly (AncientRemoveEmptyStaffContext): remove
Span_arpeggio_engraver, add Tweak_engraver to Score context.

* lily/parser.yy: reorganize file layout.

* ly/music-functions-init.ly: add tweak music function.

* lily/grob-info.cc (ultimate_music_cause): new function:
recursively lookup causes.

* lily/parser.yy (chord_body_element): allow music functions for
post-events, allow music functions for chord elements.

* lily/font-config-scheme.cc (display_fontset): add cast.

* python/convertrules.py (FatalConversionError.sub_syms): \tag
#'(a b) -> \tag #'a \tag #'b rule.

* python/musicexp.py (Output_printer.dump_version): new function