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.
Yes, but a block is semantically different from a single statement. A single return statement would also work for this conditional, but I don't really like how that looks in general.