Fretboard diagram legend improvements

[mike-spa]

Resolves: #29351

• Fixing an error in the height of the box which I had introduced here
• Removing an unnecessary layout call for chord symbols
• Fine correction to the shape of fret diagrams (was missing a half line width from the bottom). This is the cause of almost all the vtest differences btw.
• Improved/simplified calculations of chord symbols vertical positions and alignment (the combination of which was causing them to overshoot the top of the frame). This also makes sure that the 3sp row gap is honored correctly.