Add KeyboardJoypadConfiguration
GamepadButtonType: Remove from_joypad_id()

It is near to useless as it is just a cast.
RetroGtk: Move input related files into input directory
RetroGtk: Add Keyboard interface
RetroGtk: Rename some classes
Demo: Check Clutter.init()'s returned value
Demo: Add run()
Window: Treat compilation warnings