#68 add ability to display guide tones with coloration

[Robert Keller]

Requested by holger_schellwat;

A feature I like very much is the note colorization (CT, AN ...).
This made me think it should not be too difficult to add an extra
color (orange, say) for guide tones of the chord (e.g. 3 and 7).

It would be very cool to see this feature in a future version!

[Robert Keller]

There are several ways guide tones could be shown:

a. Guide tones (which are also chord tones) are colored specially when a note is entered.

b. The guide tones always show (two notes per chord), assuming some box is checked.

There is also the issue of the principal (i.e. 7-3) guide tones, vs. auxiliary (7-5) guide tones.