* lily/beam.cc (calc_direction): use default-direction

* lily/beam.cc (calc_direction): use default-direction
iso. get_default_direction()

* scm/define-grob-properties.scm (all-user-grob-properties): add
default-direction property.

* scm/define-grobs.scm (all-grob-descriptions): add MelodyItem.

* lily/stem.cc (calc_default_direction): remove
Stem::get_default_direction, use default-direction with callback
instead.

* lily/melody-spanner.cc (calc_neutral_stem_direction):

* lily/melody-engraver.cc: new file. Acknowledge stems for
interpolated stem directions.

* lily/melody-spanner.cc: new file. Interpolate stem directions.

* scm/define-grobs.scm (all-grob-descriptions): add MelodyItem

* lily/slur-engraver.cc (acknowledge_extra_object): remove
DynamicText hardcoding.