Add a class name on note heads and fret number to allow selective highlighting

# Description
This feature would add an option (toggled off by default) to automatically generate a class on every `<text>` element representing a note head on the score or a fret on the tablature.

The class name would contain both the string and fret number of the note, e.g. : `alphatab-note--4-12`. This would make it possible to easily highlight certain notes using CSS.