Make tab-note-heads and fretboards use common code.

Make tab-note-heads and fretboards use common code.

This combines the string and fret assigning code of the
tab-note-heads engraver and the fret-boards engraver.  Both
will use scheme procedures defined in scm/translation-functions.scm
and specified by context properties initialized in ly/engraver-init.ly

 * Modify the calling sequence of noteToFretFunction so that it
   takes a context, a note-list, a string-list, and an optional
   grob.  If grob is included, noteToFretFunction will add the
   fretboard to the grob.  If grob is not included, noteToFretFunction
   will return a list of (string fret finger) tuples.

 * Refactor the code in scm/translation-functions.scm to support
   this separation of function.

 * predefinedDiagramTable is now a context property for TabStaff as
   well as for FretBoards.  This means that if a chord is present in
   TabStaff, and predefinedDiagramTable for that TabStaff is not #f,
   and a predefined diagram for the chord exists, the TabStaff will
   display the notes of the predefined diagram.

 * Change tab-note-heads-engraver so that it calls noteToFretFunction
   instead of having the fret calculation code inside the engraver.

 * Change the noteToFretFunction calling sequence in fretboard-engraver
   to match the new definition

 * Add an optional default argument to ly:context-property so we
   can set a default at call time