Implementation of PointerGestures protocol

Summary:
Pointer gestures are created for a pointer and there are two types of
gestures: swipe and pinch.

At a given time there can only be one active gesture. The implementation
in SeatInterface ensures that there can only be one active gesture.

Each gesture consists of a start event, 0 to multiple update events and
an end event. The end can also be a cancel. To better support this the
implementation doesn't follow the protocol and splits end and cancel
into dedicated methods in the server side and into dedicated signals in
the client side.

Reviewers: #plasma_on_wayland

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D3169