* lily/grob.cc: remove is_empty()

* lily/grob.cc: remove is_empty()

* lily/grob-property.cc: remove add_to_list_property().

* lily/grob.cc: remove X-extent-callback / Y-extent-callback.

* lily/include/grob.hh (class Grob): remove property_callbacks_
member. Properties of procedure type are assumed to be callbacks.

* lily/grob.cc: remove set_extent_callback(), set_extent() and
has_extent_callback()

* lily/grob.cc (flush_extent_cache): rewrite.

* lily/grob.cc (Grob): don't set extent from ctor.
(extent): use property callbacks.

* lily/grob-property.cc (del_property): new function.