Digital user input and HID module implementation for PAD

These changes include:
* An implementation of HID module PAD emulation.
* A simple key mapping system which supports multiple devices and front-ends.
* The connections needed to route input from both GLFW and Qt.
Should be `key_code` per our style guide, see: https://github.com/citra-emu/citra/blob/master/CONTRIBUTING.md