Make menu button layout option configurable via config file

https://git.card10.badge.events.ccc.de/card10/firmware/-/merge_requests/330 introduced the option to swap the "select" and "scroll down" buttons. Currently apps which want to do so need to subclass the SimpleMenu class and set that option before calling the original constructor.

The SimpleMenu class should read the config menu and check which layout is to be used.