[libinput] Expose all input devices through DBus

Summary:
The Connection exposes a new service called org.kde.KWin.InputDevice
and every Device registers an own object exposing all properties.

This allows an external configuration tool to change the behavior of
the devices at runtime. E.g. to test configuration settings.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2407