Rationalize string number handling for notes and chords

Rationalize string number handling for notes and chords

Enable the use of both articulations and events for handling
string numbers, while still allowing only some notes to have
string numbers.

Create articulations.cc to house the common code used by both
tab-note-heads-engraver and fretboards-engraver to get string
numbers from the music stream.  This same code can be used
for string-bend articulations and events.

Create a regression test to demonstrate that the code will work
with string numbers partially specified.