* ly/engraver-init.ly (AncientRemoveEmptyStaffContext): use length-fraction.

* ly/engraver-init.ly (AncientRemoveEmptyStaffContext): use length-fraction.

* lily/stem.cc (calc_length): multiply with length_fraction. Makes
for easier grace tweaks.

* lily/beam.cc: remove dir-function.

* lily/system.cc (pre_processing): use callback to trigger spacing procedures.

* lily/grob.cc: remove spacing-procedure.

* lily/note-head.cc (calc_stem_attachment): new function.
(internal_print): use callback to get glyph-name.

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

* lily/grob.cc (get_stencil): simplify: use callback mechanism to
calculate stencil.
(get_print_stencil): rename from get_stencil: create stencil with
transparency, color and cause.

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

* python/convertrules.py (conv): insert temporary warning rule.

* lily/grob.cc: elucidate doc about after/before-line-breaking

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

* lily/grob.cc: change after/before-line-breaking-callback to
after/before-line-breaking dummy properties.

* lily/grob.cc (other_axis_parent_positioning): new function.
(same_axis_parent_positioning): new function

* 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.

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