* lily/grob-property.cc (get_interfaces): new function.

* lily/grob-property.cc (get_interfaces): new function.

* lily/grob-scheme.cc (LY_DEFINE): new function ly:grob-interfaces

* lily/stem.cc (calc_stem_end_position): new function.
(calc_length): new function.
document details for stem.
remove Stem::get_direction()

* lily/grob-scheme.cc (LY_DEFINE): new function ly:grob-set-callback!

* lily/grob-property.cc (set_callback): new function.

* lily/script-engraver.cc (make_script_from_event): don't trigger callback.

* scm/define-grobs.scm: change print-function to stencil callback everywhere.

* lily/grob-property.cc (try_callback): remove marker if applicable.

* lily/stem.cc (height): idem.

* lily/stem-tremolo.cc: idem.

* lily/include/grob.hh: remove Grob_status.

* lily/beam.cc (calc_direction): use pseudo-property for beam
direction callback.
(calc_positions): use callback

* lily/stem.cc (calc_stem_end_position): use callback.
(calc_positioning_done): idem.
(calc_direction): idem.
(calc_stem_end_position): idem
(calc_stem_info): idem.

* lily/grob-property.cc (get_property_data): new function:
(try_callback): new function.

* scm/define-grob-properties.scm (all-user-grob-properties): doc
callbacks property.