The extension keyboard support in XInput 1.x is hopelessly broken.
Nevertheless, it's possible to use some bits of it, as we prefer the
core keyboard events to the XInput events, thus at least having proper
handling for X11 key events on the Stage window.
Even with XInput support we should always register core devices. This
allows us to handle enter and leave events correctly on the Stage and
to have a working XInput 1.x support in Clutter.
Instead of overloading the device id of 0 and 1 we should treat the core
devices as special, and have a pointer inside the X11 backend singleton
structure, for fast access.
