Pass SwitchEvent from LibInput through KWin and add to DebugConsole

Summary:
This change introduces a new SwitchEvent and passes it through the
InputEventSpy and InputEventFilter. The DebugConsoleFilter implements it
so that the events can be monitored in the debug console.

Test Plan: Untested as my only device with such switches has too old libinput

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9521