* lily/grob.cc (get_stencil): simplify: use callback mechanism to

* 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/multi-measure-rest.cc (set_text_rods): use stencil directly.

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

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

* lily/slur-scoring.cc (enumerate_attachments): robustness fix.

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

* lily/stem-tremolo.cc: idem.

* lily/slur.cc (height): don't use get_uncached_stencil()

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

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

* lily/slur-scoring.cc (calc_control_points): use callback

* lily/slur.cc (calc_direction): use callback.

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

* 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.