2 Enhancements for Custom Mode Widget

1. Print the "key" somewhere on the widget. Currently, the mode is shown (e.g. locrian, dorian, etc) but not the key (e.g. a b c d e f F# etc). I recommend to print the key next to where the mode is shown.

2. It would be nice if the play button changed into a stop button during its playback, so the user could stop playback midway if desired.