* lily/include/directional-element-interface.hh:

* lily/include/directional-element-interface.hh:
junk class, make {get,set}_grob_direction functions.

* lily/slur.cc (get_attachment): do something sensible when
attaching to stem without heads (eg. a rest).

* lily/stem.cc: remove #'up-to-staff ; use #'stem-end for forcing
up to staff. Add #'rests.
remove #'support-head property
(brew_molecule): small cleanup.
(flag): remove #'grace property.