* lily/rest.cc (y_offset_callback): use offset callback

* lily/rest.cc (y_offset_callback): use offset callback
iso. after-line-breaking callback.

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

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

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

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