Rework computer keyboard input to be mapped piano keyboard

Instead of using computer keyboard letter names for piano note names,
(which allowed only entering natural notes), we now map the piano
keyboard onto the computer keyboard. The naturals are on the home row,
(middle C is on 'A' and the next higher C is on 'K'), and the sharps
map onto the top row in the obvious position, (C# on 'W, D# on 'E',
etc.).