Adds alt+⇧ Shift+? as an access key to show a list of all assigned access keys.

Credit

Guarapiranga had the idea, NguoiDungKhongDinhDanh wrote the first version, in which the cheat sheet was static, and Nardog provided the second version, in which the cheat sheet varies from page to page and from user to user, depending on which access keys are assigned where and for whom.