Use a DeviceManager sub-class similar to the Win32 backend one, which
creates two InputDevices: a core pointer and a core keyboard.
The event translation code then uses these two devices to fill out the
.device field of the events.
Throw in enter/leave tracking, given that we need to update the device's
state.
http://bugzilla.clutter-project.org/show_bug.cgi?id=2490
