change driving direction: only when recording
buttons: only execute function when rendered

- buttons that are rendered based on a condition won't execute their
respective function while they are not rendered
can't generate course when already generated

"generate course" can only be executed when:
- no course has already been generated
- the current (loaded/recorded) course has more than 4 waypoints
- a valid starting corner has been set
- a valid starting direction has been set
- there is only one course loaded (no combined courses)

Existing bugs:
-when loading a saved generated course from course management, or when
that course is already loaded upon starting the save game, "generate
course" can be executed