* scm/translation-functions.scm (determine-frets-mf): new

* scm/translation-functions.scm (determine-frets-mf): new
function: compute fret numbers.

* scm/output-lib.scm (string-frets->description) new function.
(fret-board::calc-stencil): new function

* scm/fret-diagrams.scm (fret-diagram-verbose): update doc string.

* scm/define-grobs.scm (all-grob-descriptions): add FretBoard grob.

* lily/include/lily-guile.hh (ly_cxx_vector_to_list): new function.

* lily/tab-note-heads-engraver.cc: cleanups.

* lily/fretboard-engraver.cc: new file

* ly/engraver-init.ly: add FretBoards context