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

[Datseris]

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.