* lily/include/translator.icc: new file.

* lily/include/translator.icc: new file.

* lily/paper-column-engraver.cc (process_music): new
file. Separate Paper_column factory from Score_engraver.

* lily/vertically-spaced-context-engraver.cc: administer
spaceable-staves property of System.

* lily/translator-group-ctors.cc: new file.

* lily/context-property.cc (make_grob_from_properties): construct
the right Grob class programmatically, looking at the class entry
for the meta property.

* lily/break-align-engraver.cc (stop_translation_timestep): call
Break_align_interface::add_element() directly.

* lily/context.cc (measure_position): measure_position() is now a
normal function.

* lily/include/translator.hh (class Translator): rename
process_acknowledged_grobs() to process_acknowledged() and move to
Translator.

* scm/define-grobs.scm (all-grob-descriptions): add a 'class meta
field for each grob description.

* lily/include/translator-group.hh (class Translator_group):
change to base class. Separate class from Translator. This gets
rid of virtual inheritance for Engravers/Performers.

* lily/staff-performer.cc (class Staff_performer): derive
Staff_performer from Performer, not Performer_group_performer

* Lily/translator-group.cc (precomputed_recurse_over_translators):
new function.
(precompute_method_bindings): new function. Precompute lists of
Translators, so we only call methods (process_music,
start_translation_timestep, etc.) for Translators needing
it. Also: dispose of pointer-to-member-function calls.

* lily/engraver-group-engraver.cc: remove engraver_each,
recurse_down_engravers ()

* lily/note-head.cc (internal_print): only call
glyph-name-procedure if  style != default. 3 %  speed increase (wtk2-fugue1).