Summary:
This change implements the zwp_relative_pointer_v1 protocol which allows
to send relative motion events.

The (unstable) protocol consists of a RelativePointerManager which
creates RelativePointers for a given Pointer. This interface currently
only has one event to report the relative motion. It carries the delta,
the non-accelerated-delta and a timestamp in microsends granularity.

On the server side the implementation is mostly internal. Once a
RelativePointerManagerInterface is created one can send relative motion
events through the SeatInterface. The SeatInterface takes care of
sending it to the responding RelativePointerInterface. The protocol does
not restrict the sending of "normal" and relative motion events. Thus it
can be combined in any way one wants. This allows to have a rather
simple implementation. A user of the SeatInterface can just start to
feed the relative motion events (if the information is available) to the
SeatInterface together with the pointer events.

On client side a new RelativePointerManager and RelativePointer class
are added. The RelativePointerManager creates the RelativePointer for a
given Pointer. The event sent to RelativePointer is transformed in a
normal signal.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D2978