Summary:
Gesture events are swipe or pinch events on a touch pad.

This change implements basic support by:
 * wrapping them in LibInput::Event
 * processing them in LibInput::Connection and emitting
   dedicated signals
 * Forwarding them in InputRedirection to PointerInputRedirection
 * Support them in the internal input event filter
 * Printing debug information in DebugConsole

Further handling is not yet done. In future the following should be
implemented:
 * activating e.g. zoom and present windows on pinch/swipe gesture
 * forwarding non global gestures to KWayland

Note that forwarding to KWayland is not yet useful as QtWayland does
not yet have support for the unstable protocol. No Qt application could
make use of it yet. So for the moment just global gestures is the best
we can get.

Test Plan: Looked at output of DebugConsole when triggering gestures

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D2359