* lily/line-interface.cc (line): remove thickness from calling interface.

* lily/line-interface.cc (line): remove thickness from calling 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/lookup.cc: remove Lookup::horizontal_slope (): it is
functionally identical to beam.