Feature: use libconfig to load a config file to set VID/PID of mouse/keyboard

[Gadgetoid]

Since libconfig is already a dependency and provides some nice config file handling, I propose the following config file format:

# Pi 400 Keyboard/Mouse Config File

version = "1.0";

application:
{
  keyboard:
  {
    vid = 0x04d9;
    pid = 0x0007;
    device_path = "/dev/input/by-id/usb-_Raspberry_Pi_Internal_Keyboard-event-kbd";
  };

  mouse:
  {
    vid = 0x093a;
    pid = 0x2510;
    device_path = "/dev/input/by-id/usb-PixArt_USB_Optical_Mouse-event-mouse";
  };
};

A path to the config file could be passed to pi400kb on the command-line.