x11: Always handle core device events before XI events

x11: Always handle core device events before XI events

The XI 1.0 layer is complementary to the X11 core devices handling; this
means that core events will still be emitted for the core pointer and
keyboard devices, and that secondary (floating) devices should be
handled on top of that.

Thus, the XI event handling code should be executed (if explicitly
compiled in and enabled) if the core device events have not been parsed.

Note: this is going away with XI2, which completely replaces both core and
XI1 events.