* lily/stem.cc (thickness): new function.

* lily/stem.cc (thickness): new function.

* lily/staff-symbol.cc (get_ledger_line_thickness): new function:
compute thickness from ledger-line-thickness.

* lily/note-head.cc (brew_ledger_lines): use
Staff_symbol::get_ledger_line_thickness ().

* lily/beam.cc (rest_collision_callback): use
Staff_symbol_referencer::staff_space().

* lily/parser.yy (My_lily_lexer): change syntax: \newpartcombine
Mus1 Mus2

* lily/include/translator.hh (class Translator): add accepts_list_
so  \with { \denies / \accepts } also works.

* scm/music-functions.scm (determine-split-list): determine split
list from music events.

* lily/new-part-combine-iterator.cc: more states.

* lily/moment.cc (LY_DEFINE): new function ly:moment<?