The device manager now fully owns the GSources corresponding to the
devices it manages. This will allow not only to remove the source when
udev signals a device removal but also handle read() errors gracefully
by removing the faulty device from the manager.
This backend is a event backend that can be enabled for EGL (for now).
It uses udev (gudev) to query input devices on a linux system, listens to
keyboard events from input devices and xkbcommon to translate raw key
codes into key keysyms.
This commit only supports key events, more to follow.