Display keyboard shortcuts on pressing `?` and with button on the GUI

The keyboard shortcuts are awesome! Specifically the ones that change tempo and pitch!

However, sometimes one can forget which key does what. It would be to have a way to see the shrotcuts from within the GUI of Mplay.

This is a request to somehow show the shortcuts on pressing `?` and/or have a button with maybe `?` as a sign, that shows the shortcuts and info about the application.