backends/x11: Call meta_backend_update_last_device() from XI_DeviceChanged

On X11, calling this function on meta_display_handle_events() will not catch
mouse events happening over clients, so poke directly in the backend for
XI_DeviceChanged events, which mutter will get on device switches.

The code has been slightly refactored so we deal with XIEvents at a single
handle_input_event() function.

https://bugzilla.gnome.org/show_bug.cgi?id=712775