* lily/hairpin.cc: use Line_interface

* lily/hairpin.cc: use Line_interface

* lily/line-interface.cc: new file. Line_interface for everything
with dotted/dashed lines.

* lily/tuplet-bracket.cc (make_bracket): use Line_interface.

* lily/ottava-bracket.cc (brew_molecule): new file, separate routine
for constructing ottava brackets.

* lily/ottava-engraver.cc (acknowledge_grob): hang the bracket on
the note heads.

* Documentation/user/internals.itely (Engravers and performers):
fix broken link.

* scm/music-functions.scm (make-mark-set): new function.

* lily/lookup.cc: remove Lookup::horizontal_slope (): it is
functionally identical to beam.

* lily/mensural-ligature.cc (brew_flexa): replace horizontal_slope
() with beam ().

* Documentation/user/refman.itely (Rehearsal marks): document new
functionality.