Support for relative pointer motion events

Summary:
If KWin interacts with Libinput the RelativePointerManager interface
gets created on the Wayland server. The ForwardInputEventFilter does
forward the relative motion events in addition to the normal motion
events.

In order to properly support the relative motion events as they are
expected by the Wayland protocol the handling of pointer motion events
got slightly adjusted:
* Libinput Pointer event extended by the additional data points
* Libinput Pointer event carries the delta as a QSizeF instead of
QPointF
* PointerInputRedirection adjusted to take a pointer motion event with
more arguments
* Custom QMouseEvent subclass adjusted to carry the additional members

The DebugConsole is adjusted to show the relative motion events in
addition to the global position.

Test Plan:
Verified the manager object is created and verified the
events in DebugConsole. Unfortunately not aware of any test application.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D2979