* lily/parenthesis-engraver.cc (acknowledge_grob): don't do

* lily/parenthesis-engraver.cc (acknowledge_grob): don't do
set_parent() for parentheses. This will make it show up in bbox
calculations.

* scm/output-lib.scm (parentheses-item::print): rewrite, using
ly:grob-common* functions.

* lily/grob-scheme.cc (LY_DEFINE):
ly:grob-common-refpoint-of-array: new function
ly:grob-common-refpoint: new function
ly:grob-relative-coordinate: new function

* lily/instrument-name-engraver.cc (process_music): use
Text_interface::is_markup(). This fixes \markup on instrument names.

* scm/define-grobs.scm (all-grob-descriptions): set line-thickness
for ties and slurs.

* lily/slur.cc (print): idem.

* lily/tie.cc: add line-thickness for ties.