Implement the relative pointer protocol

Implement the relative pointer protocol

A wp_relative_pointer object is an extension to the wl_pointer interface
only used for emitting relative pointer events. It will only emit events
when the parent pointer has focus.

To get a relative pointer object, use the get_relative_pointer request
of the global wp_relative_pointer_manager object.

The relative pointer protocol is currently an unstable protocol, so
unstable protocol naming conventions has been applied.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>