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