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.
Since you touched this, could have been renamed to `render_window` to clean this up with the style guide. Just an FYI, doesn't necessarily need to be changed.