* lily/rest.cc (y_offset_callback): merge function of 3 callbacks.

* lily/rest.cc (y_offset_callback): merge function of 3 callbacks.

* lily/grob.cc (y_parent_positioning): remove axis argument from
parent_positioning callbacks.
(Grob): junk init code
(get_offset): use generic property callbacks for
X-offset, Y-offset. Remove Grob::{add,has}_offset_callback.
(axis_parent_positioning): new function
(axis_offset_symbol): new function
(chain_offset_callback): new function
(add_offset_callback): new function

* lily/beam.cc (rest_collision_callback): change signature; this
is now a chained callback.

* lily/include/dimension-cache.hh (class Dimension_cache): remove
callback administration.

* lily/simple-closure.cc: new file. Smob type that allows "grob"
member functions

* lily/slur-engraver.cc (acknowledge_extra_object): use chained
callbacks.

* lily/slur.cc (outside_slur_callback): take 2 arguments.

* lily/rest.cc (polyphonic_offset_callback): remove.

* lily/script-engraver.cc (try_music): remove bool* argument.

* scm/script.scm (default-script-alist): set quantize-position
appropriately.

* lily/script-engraver.cc: remove follow_into_staff_ special coding.

* lily/side-position-interface.cc (aligned_side): move staff
position quantization from
Side_position_interface::quantised_position().
(quantised_position): remove