Use input_context in main_menu.cpp

Rather big, but does not change behavior. It removes direct input via `getch` and `get_input` and uses the keybindings from the input manager instead.

The menu shortcuts are still there and take precedence over the other input, which means even if "move left" is bound to 'n', pressing 'n' will bring the user to the "new game" screen instead of moving the selection to the left.