Improve chord symbol + fretboard diagram interaction

[bkunda]

[Updated by @mike-spa]

Fretboard diagram and chord symbol are in a parent-child relationship with the diagram being the parent, as evidenced by the fact that moving the diagram also moves the chord with it, but moving the chord doesn't move the diagram. The same pattern should apply to other interactions:

1. Copy-pasting the chord just pastes the chord, while copy-pasting the diagram copy-pastes both.
2. Cut-pasting: same.
3. Delete: same.
4. Toggling visibility: same.