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.
Given that this is encapsulated in an interface (as it should be), this should be static and hence called `key_map` or `s_key_map` instead.